This is arguably the pick of the group-stage fixtures, as the 2010 tournament winners face the reigning European champions in Sochi.
La Roja head into the competition in a state of disarray, however, having parted company with manager Julen Lopetegui two days before this mouth-watering opening fixture.
Follow all of the action as it unfolds with our extensive updates below.
PORTUGAL: Rui Patricio; Cedric, Pepe, Fonte, Guerreiro; Carvalho, Moutinho; Fernandes, B.Silva, Guedes; Ronaldo
SUBS: Alves, M Fernandes, A.Silva, Joao Mario, Lopes, Ruben Dias, Ricardo, Gelson, Mario Rui, Quaresma, Beto, Adrien
SPAIN: De Gea; Nacho, Pique, Ramos, Alba; Busquets, Koke; Silva, Isco, Iniesta; Diego Costa
SUBS: Carvajal, Saul, Rodrigo, Thiago, Lucas, Odriozola, Arrizabalaga, Azpilicueta, Monreal, Iago Aspas, Asensio, Reina
How on earth do you go about analysing a game like that?! Cristiano Ronaldo's 51st career hat-trick is the headline news from Sochi - it was also coincidentally the 51st treble scored in World Cup finals history - but there is so much more than just that to take away from this match. Diego Costa also netted twice and Nacho scored a screamer, while David de Gea's howler for Ronaldo's second goal has almost been forgotten. Thanks for joining our joining live coverage!
90+4 min FULL TIME: PORTUGAL 3-3 SPAIN
90+2 min Four minutes have been added on at the Fisht Olympic Stadium, and we have now played half of those. Quaresma takes the ball past a couple of players but his shot is blocked when the goal opened up for him. Ronaldo then glanced a header wide.
90 min The 90 minutes are up in Sochi, but everyone in the ground and watching around the world would happily take another couple of hours of this. Ronaldo's dipping free kick is the icing on the cake of what has been an unbelievable match.
88 min Put this match down in World Cup folklore! Cristiano Ronaldo sends the free kick up and over the wall to complete his hat-trick - the 51st in World Cup finals history, and also the 51st of his professional career. What a game!
87 min GOAL! PORTUGAL 3-3 SPAIN (CRISTIANO RONALDO)
86 min SPAIN SUB! Lucas Hernandez has been brought on for David Silva to help close out the final stages of thos match. Danger for Spain, though, as a free kick has just been conceded 25 yards from goal...
84 min SAVE! Iago Aspas gets a shot on goal that is hit far too central, allowing Rui Patricio to gather. That came at the end of another impressive passing move from Spain, who are doing well to close this game out.
82 min PORTUGAL SUB! Cedric Soares with a decent delivery over the top for Cristiano Ronaldo, who could not get to the ball before Gerard Pique. The centre-back was clattered by a couple of other players and is now receiving treatment. During the stoppage, Andre Silva has been brought on for Guedes.
80 min Both managers have one more substitute available to make, and I'd be surprised if Andre Silva is not one of those brought on for Portugal. Stand-in Spain boss Hierro is more likely to shuffle his pack in midfield than up top, on the other hand.
78 min SPAIN SUB! Ricardo Quaresma gets a shot away but it lacked quality and was never going to test David de Gea. Iago Aspas has just been brought on for the final 12 minutes, taking over from two-goal striker Diego Costa.
76 min This game really has flown by - less than 15 minutes left to play in Sochi! It has undoubtedly been the game of the tournament so far, more than living up to its pre-match hype. Five goals so far and still plenty of time for another.
74 min Portugal starting to make chances of their own, but there has been nothing clear-cut for a while. At the risk of cursing Spain, Cristiano Ronaldo has been a little quiet of late... There does look to be a sixth goal in this match.
72 min CHANCE! Diego Costa misses a big chance to score a World Cup hat-trick. The ball was cut back to the striker in the middle of the box, but he dragged the subsequent shot wide of Rui Patricio's goal.
70 min SUBS! Spain make their first change of the evening, bringing on Thiago Alcantara for the tiring Andres Iniesta in central midfield. Portugal have made their second alteration, meanwhile - Ricardo Quaresma is on for Bernardo Silva.
68 min PORTUGAL SUB! Joao Mario has finally been introduced in place of Bruno Fernandes. A few different options on the bench for Santos to turn to, including Andre Silva, but his first roll of the dice comes in the middle of the park.
66 min Portugal have been made to wait a couple of minutes to make that change as the ball has yet to go out of play! Spain had been playing the better football but that has changed over the past couple of minutes; the Selecao now on top.
64 min Bernardo Silva with a couple of stupid challenges but he avoids being booked by the referee. Portugal boss Fernando Santos is now readying Joao Mario to come on, with that change likely to be made in the next couple of minutes.
62 min That Nacho goal really was special. The ball bounced just in front of him yet he made perfect contact, sending it in the only place that Rui Patricio could not reach. How will Portugal react to Spain's quickfire double?
60 min Spain edge in front for the first time tonight through an unbelievable Nacho strike! Guilty for giving away the penalty for the first goal, the full-back put his boot through the ball from 20 yards out and sent it flying past Rui Patricio.
59 min GOAL! PORTUGAL 2-3 SPAIN (NACHO)
58 min For all of Spain's impressive build-up play today, their first goal came via a Busquets punt forward and the second via a headed knockdown. A little over half an hour to play here and it is very much all to play for - good luck calling this one!
56 min Well that is the equivalent of the commentator's curse! I'd just mentioned how Spain had failed to really get going since the restart, and right on cue Diego Costa bags his second of the evening. A deep free kick was headed back across goal by Sergio Busquets, with the striker there to tap home from close range.
55 min GOAL! PORTUGAL 2-2 SPAIN (DIEGO COSTA)
54 min Plenty of 'almost' moments for the 2010 World Cup winners in the opening 10 minutes of this second half. It is not quite happening for Hierro's charges at the moment, though, and opposite number Santos will be pleased with how things are panning out.
52 min As you can see from the stats here, Spain are in complete control when it comes to possession of the ball. La Roja have looked lively in the final thrid, too, finding the target three times.
50 min The second half has not taken off in the same manner as the first just yet. Both sides looking a little disjointed, which will obvious suit Portugal a lot more than their opponents, who are chasing a goal or two in the remaining minutes.
48 min Ronaldo now has 22 goals in his last 18 outings for Portugal, and remains on course for a World Cup hat-trick. The Real Madrid ace and teammate Bernardo Silva have both slipped on the surface, which looks pretty wet after being heavily watered.
46 min RESTART! Portugal get us back under way at the Fisht Olympic Stadium, sitting comfortably thanks to star man Cristiano Ronaldo's double. Neither manager has made a change at the interval, so it is as you were.
Portugal take a 2-1 lead into half time thanks to a brace from Cristiano Ronaldo. It has been a thrilling opening 45 minutes at the Fisht Olympic Stadium, with Spain striker Diego Costa netting either side of Ronaldo's two goals.
45+2 min HALF TIME: PORTUGAL 2-1 SPAIN
45 min The best goalkeeper in the world has just had an absolute howler! Cristiano Ronaldo, with barely any backlift, gets a shot away from 20 yards which went right through the hands of David de Gea. Portugal will take a one-goal lead into halftime.
44 minGOAL! PORTUGAL 2-1 SPAIN (CRISTIANO RONALDO)
43 min SAVE! Not the cleanest of strikes from Spain's star performer in Isco, who made some space on the edge of the area and got a shot away at the end of the move. Rui Patricio got behind it without having to break stride.
41 min One of Portugal's better spells on the ball over the past couple of minutes, keeping hold of possession but not really doing a great deal with it. La Roja look some incredibly composed as soon as they win it back, going from back to front with ease.
39 min There has been no change to the flow of the match - Spain are dominating possession, and looking lively every time they get into the final third, while Portugal are dangerous on the counter. Six minutes to go of a gripping first half.
37 min CLOSE! Spain have played some brilliant football today. Another impressive move culminates in Koke pulling the ball back for Andres Iniesta, whose shot ended inches wide of the far post after taking a touch on its way through.
35 min A controversial penalty call, a trademark goal from Diego Costa, a bullet of a strike against the crossbar and a couple of decent chances missed - it has been the best first half of the World Cup so far, and we still half 10 minutes to go before the interval.
33 min A little over half an hour played in Sochi and Spain are the side in control of possession. That has been the case pretty much throughout, although Portugal look very dangerous when given the chance to counter-attack.
31 min Well today's first two games were a little boring, to say the least, so thank goodness for this thriller being staged by Portugal and Spain. Just the two goals so far, but talking points aplenty and we will surely see that tally added to.
29 min YELLOW CARD! On the incident in the build-up to Spain's equaliser, Costa did make contact with Pepe but the referee was justified in allowing play to continue. Bruno Fernandes has just joined Sergio Busquets in the referee's book for a strong challenge.
27 min OFF THE BAR! This match is living up to the hype - and then some! Isco with an absolute belter of a shot from distance, which hit the underside of the crossbar. The referee's watch informed him that it did not cross the line, despite Isco's protests.
25 min A superb individual goal from Diego Costa! The Atletico Madrid used his strength to hold off his man and, after delaying the shot for what felt like an age, he picked out the bottom corner of Rui Patricio's net. VAR is required as Costa was a little rough when winning the ball off Pepe in the build-up, but the goal has been awarded.
24 min GOAL! PORTUGAL 1-1 SPAIN (DIEGO COSTA)
23 min CHANCE! Brilliant play from Iniesta in his final international tournament. The ball was cut back for David Silva, whose shot was deflected wide. Nothing came from the corner and, after mounting a counter-attack, Guedes should have done a lot better when picked out in the middle - a poor first touch denied him the chance of a proper shot, with Alba getting a block in.
21 min YELLOW CARD! Sergio Busquets is the first player of the evening to be shown a yellow card after following through on Goncalo Guedes. Cristiano Ronaldo, with one goal to his name already, blasts the free kick into the wall.
19 min Guedes, a real star in the making, has half the length of the field to run as Portugal are given another chance to counter. The youngster never looked comfortable up against Ramos, though, and was forced into a blocked shot from 30 yards.
17 min I 've already change my mind once with that Ronaldo penalty call and I'm tempted to revert back to my original decision. Nacho did make contact with his Real Madrid teammate, but Ronaldo knew exactly what he was doing and made the contact first.
15 min La Roja starting to dominate the game now and, unlike in the previous 10 minutes or so, they have popped up in dangerous zones over the last five minutes. That could suit Portugal - they have the perfect man in Ronaldo to lead their counter.
13 min With that early goal, Ronaldo becomes only the fourth player to score in four World Cups, after Uwe Seeler, Pele and Miroslav Klose. Does he have another tournament in him? Possibly, but his powers will surely wane before then.
11 min SHOT! Spain's first half-chance to level up the contest falls the way of David Silva who, despite being in good goalscoring form for his national side this season, cannot keep the awkward volleyed attempt on target.
9 min Spain with plenty of the ball since falling behind, as you would expect, but so far they have been unable to really get it into the final third. Portugal with just the one attack so far, culminating in the move that led to Ronaldo's penalty.
7 min Any chance of Spain being given a chance to settle into the match has certainly gone with that Ronaldo penalty. Again, though, there is plenty of experience throughout this Spanish side and it will take more than that to knock them off their feet.
5 min In his 36th outing at a major tournament, Cristiano Ronaldo gives Portugal the lead from the penalty spot. The Real Madrid superstar stuck the ball to David de Gea's left; the keeper went the opposite way. After seeing a replay, the referee did appear to make the right call.
4 min GOAL! PORTUGAL 1-0 SPAIN (CRISTIANO RONALDO, PEN)
3 min PENALTY TO PORTUGAL! Explosive start to this match! Cristiano Ronaldo was sent to the ground by Nacho following a couple of step-overs. A penalty is awarded, but to me it looked like the Real Madrid ace made the contact with his clubmate rather than the other way around.
1 min KICKOFF! Spain, sporting an all-white strip in Sochi this evening, get us under way in the pick of the group-stage fixtures. Portugal, the reigning European champions, are looking to make a fast start at the Fisht Olympic Stadium.