In the latest of pro-athlete epiphanies on police violence and a willingness to be vocal about it, tennis superstar Serena Williams made her feelings clear about her fears about what can happen in a split second to an innocent person, and it came to her while she was riding in a car with her nephew.

Writing in an emotional Facebook post on Tuesday evening, Williams, who turned 35 on Monday, expressed her apprehension about her nephew being confronted by police.

Similar sentiments have been expressed in the pro-sports world, most notably by Colin Kaepernick, who has knelt at the sidelines during the national anthem at each San Francisco 49ers game this season, and the Denver Broncos’ Brandon Marshall, who has joined him in the gesture.

But Williams’ may be the most prevalent voice because she is among the most recognizable athletes in the world, and has entry into circles around the globe that many others do not.

It also brings the discussion about police violence to a different environment because in sports like basketball and football the issue has been discussed for decades. But Arthur Ashe and Althea Gibson are the only notable people in tennis known to have historically discussed civil rights openly. Even when James Blake was mistakenly pummeled by New York police last year, the conversation faded away.

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“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,” she wrote.

Related: Is Serena Williams the GOAT? Is That Even The Right Question?