Serena Williams vows “I will not be silent” in a social media post addressing police shootings of African American men.

The tennis star wrote:

Today I asked my 18 year old nephew (to be clear he’s black) to drive me to my meetings so I can work on my phone #safteyfirst. In the distance I saw cop on the side of the road. I quickly checked to see if he was obliging by the speed limit. Th[e]n I 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. 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.”

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.

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 realized we must stride on—for it’s not how far we have come but how much further still we have to go.

I th[e]n wondered th[e]n have I spoken up? I had to take a look at me. What about my nephews? What if I have a son and what about my daughters?

As Dr. Martin Luther King said, “There comes a time when silence is betrayal.”

The statement comes in the wake of increasing political activism in sports. This summer LeBron James, Chris Paul, Carmelo Anthony, and Dwyane Wade called for athletes to “step up and take charge” in demanding “change.” Colin Kaepernick and a handful of NFL players protest police shootings of African Americans by refusing to stand for the “national anthem” before games.

Williams, who boasts 22 major titles in singles competition, famously boycotted Indian Wells four 14 years after enduring jeers and racist comments when her sister Venus defaulted to her because of injury before a semifinal match. Williams acknowledged five years ago, “Tennis is a very white sport.”

Williams offered as a complimentary close, “I will not be silent.”