Serena Williams has written a heartfelt Facebook post about police shootings of black people in the USA.
Williams said she was inspired to speak out after thoughts she had as a passenger in her 18-year-old nephew's car.
Passing a police officer, Williams said she checked to see if her nephew was driving within the speed limit, so as to avoid an interaction with the officer. In that moment she said she found herself remembering "that horrible video of the woman in the car when a cop shot her boyfriend".
The video Williams refers to was filmed in Minnesota earlier this year. Philando Castile, 32, was shot and killed by police while his girlfriend Diamond Reynolds sat in the passenger seat. Reynolds said he was just reaching for his licence and registration when the officer shot him four times.
"All of this went through my mind in a matter of seconds," said Williams. "I even regretted not driving myself. I would never forgive myself if something happened to my nephew. He's so innocent. So were all 'the others'.
"Why did I have to think about this in 2016? Have we not gone through enough, opened so many doors, impacted billions of lives? But I realised we must stride on - for it's not how far we have come but how much further still we have to go.
"As Dr Martin Luther King said: 'There comes a time when silence is betrayal.'
"I won't be silent."
Williams' comments come as a strong movement of athletes has spoken out about social injustice in the USA. Colin Kaepernick, the San Francisco 49ers quarterback, has taken to kneeling before each game when the national anthem is played in protest. Many of his sporting counterparts across the USA have followed the trend.