RIO DE JANEIRO, BRAZIL - AUGUST 07: Serena Williams of the United States during her match against Daria Gavrilova of Australia in their first round match on Day 2 of the Rio 2016 Olympic Games at the Olympic Tennis Centre on August 7, 2016 in Rio de Janeiro, Brazil. (Clive Brunskill/Getty Images)

Recent incidents in which police officers have shot and killed black men have gained widespread attention, including in the world of sports. Not all athletes are emulating the national anthem protests of Colin Kaepernick, but many, particularly African Americans, are speaking up about their concerns about racial injustice, and Serena Williams added to that conversation Tuesday.

In a message posted to Facebook, the tennis superstar shared her fears of what might happen in a traffic-stop situation, if not for herself than certainly for her 18-year-old nephew. Asking why she had to worry about such a thing in this day and age, Williams cited Martin Luther King Jr. in vowing to avoid “silence” on the issue.

Williams said that she asked her nephew to drive her to some meetings Tuesday so that she could get some work done with her phone. “In the distance I saw [a] cop on the side of the road,” she wrote, and she “quickly checked” to make sure he was obeying the speed limit.

“I remembered that horrible video of the woman in the car when a cop shot her boyfriend,” Williams wrote, likely referring to the July incident in which a Minneapolis-area police officer shot Philando Castile after a traffic stop. Castile’s girlfriend, Diamond Reynolds, was in the car, along with her young daughter, and recorded the aftermath in a video that went viral and provoked sympathetic responses from President Obama and the governor of Minnesota, among many others.

“All of this went through my mind in a matter of seconds,” Williams wrote. “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.”

Williams’s story was a reminder that African American athletes who have reached the pinnacles of their sports still exist in a world that at least overlaps with the one filled with everyday struggles and injustices. LeBron James, who, like, Williams, rose from poverty to achieve enormous wealth and fame through sports, spoke of similar concerns Monday.

“You see these videos that continue to come out, it’s a scary-a– situation that if my son calls me and said if he got pulled over, that I’m not that confident that things are gonna go well and my son is going to return home,” James told reporters at the Cavaliers’ media day. “My son just started the sixth grade.”

“I’m not here to ramp on America, that’s not me,” James added. “I’m not a politician, but I’ve lived this life and I’ve got a family, and what scares me is my kids growing up in this society right now, where innocent lives are being taken and it seems like nothing is being done.”

Both James and Williams said that they were not trying to paint all police officers, or any other segment of society, as racists. “I’m not up here saying all police are bad, because they’re not. I’m not up here saying that all kids are great and all adults are great, because they’re not,” James said. “But at the same time, all lives do matter.”

“I am a total believer that not ‘everyone’ is bad,” Williams wrote on Facebook. “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?” she continued. “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.”

Saying, “I had to take a look at me,” Williams mused about whether she had “spoken up” enough. “What about my nephews? What if I have a son and what about my daughters?” she wrote.

Williams has had occasion to call out incidents of race- or gender-based bias. In March, she criticized the CEO of the Indian Wells tournament for sexist comments that quickly caused him to resign that post. She had only resumed playing in that same event the year before, following a 13-year absence prompted by alleged racist treatment of her, her sister Venus and their father by fans.

In 2015, Williams also used Twitter to react to the killing of Christian Taylor, a black college football player in Texas, by a white police officer. “Are we all sleeping and this is one gigantic bad nightmare?” she wrote.

In Tuesday’s Facebook post, Williams wrote, “As Dr. Martin Luther King said, ‘There comes a time when silence is betrayal.’ ” She concluded with the words: “I won’t be silent”