Serena Williams has voiced her fears about police violence, writing on Facebook: "I won't be silent."
The US tennis star posted that she found herself feeling wary of law enforcement during a recent journey with her teenage nephew.
Williams said that when they drove by a patrol car she remembered a woman whose boyfriend was fatally shot by police.
She is the latest athlete to speak out about friction between law enforcement and the black community in the US.
In Tuesday's Facebook post, Williams, 35, wrote that she had asked her 18-year-old nephew to drive her somewhere so she could work during the journey.
She said that when she saw a police car she "remembered that horrible video of the woman in the car when a cop shot her boyfriend". "All of this went through my mind in a matter of seconds," she added.
Williams was apparently referring to the 6 July fatal shooting of Philando Castile, whose girlfriend live-streamed the confrontation with police in St Paul, Minnesota.
The Grand Slam champ said she checked her nephew was not speeding so they would not get pulled over, and then regretted not having driven herself.
"I would never forgive myself if something happened to my nephew," Williams wrote. "He's so innocent. So were all 'the others.'"
"I am a total believer that not 'everyone' is bad. It is just the ones that are ignorant, afraid, uneducated, and insensitive that is affecting millions and millions of lives."
Williams quoted Martin Luther King Jr, writing: "There comes a time when silence is betrayal."
She added: "I Won't Be Silent".
Williams spoke out amid a wave of anti-police brutality sporting protests in the US, triggered after San Francisco 49ers quarterback Colin Kaepernick refused to stand for the national anthem.
Some 214 black people have been killed by US police this year out of a total of 821 people, according to Black Lives Matter monitoring group, Mapping Police Violence.