Barcelona lost Lionel Messi to injury as they were held to a 1-1 La Liga draw by Atletico Madrid at Camp Nou on Wednesday.
The champions knew that a win would take them to within a point of leaders Real Madrid after their draw with Villarreal, but Ivan Rakitic's first-half header was cancelled out by Angel Correa in the second period.
Perhaps more damaging for Barca was the loss of Messi to a possible recurrence of an existing groin problem, while Sergio Busquets was also withdrawn before the hour mark.
Luis Enrique's side, unbeaten in 12 league games against Atletico, had controlled much of the proceedings in the first half but found it difficult to muster clear-cut chances, before Rakitic headed in Andres Iniesta's cross from a short corner.
Atleti made a strong start to the second period and snatched the equaliser when substitutes Fernando Torres and Correa combined within a minute of coming on, just after Messi had failed to shake off discomfort sustained after a tangling with Diego Godin.
Barca won three of their four meetings with Atleti last season 2-1 and they pushed hard for a late winner, but Diego Simeone's side kept them at bay with relative ease in the closing stages to stay just a point behind in the table, and four adrift of Madrid.
Barca's early possession yielded few clear opportunities, Messi curling into Oblak's arms from 20 yards out with his first real sight of goal, and it was Atleti who conjured the better openings in the first half-hour.
Yannick Carrasco had Ter Stegen at full stretch with a curling drive from the left of the penalty area, before Filipe Luis blasted over the crossbar from 20 yards after Barca failed to clear their lines.
Neymar bent an effort narrowly wide of Oblak's left-hand post, but just as Barca's frustrations began to grow, they seized the advantage with four minutes left of the half.
Iniesta collected the ball to the left of the penalty area, and his pinpoint cross towards the far post was glanced into the bottom corner off the head of the arriving Rakitic.
Atleti came out flying after the break, with Antoine Griezmann testing Ter Stegen from close range before a timely intervention from Gerard Pique denied Kevin Gameiro a chance in the six-yard box.
Busquets was withdrawn for Andre Gomes just six minutes into the half, and it got worse for Barca when Messi was forced off just shy of the hour mark after failing to shake off what appeared to be a recurrence of a groin problem sustained early this month.
A terrible two minutes for Barca was compounded when Atletico snatched their equaliser. Torres fed fellow substitute Correa 30 yards from goal, and when Javier Mascherano lost his footing, the young striker raced clear into the area before slotting the ball beyond Ter Stegen and in off the keeper's right-hand post.
The home crowd were deflated but Barca pushed on for a winner, with Neymar twice denied by good blocks from Oblak and Pique heading narrowly wide, but Atletico held firm to move back into fourth, behind Barca and early pace-setters Madrid and Sevilla.