The sight of a police car by the roadside during a car journey with her teenage nephew Tuesday prompted a chain of thoughts that led to her emotional post.
"I quickly checked to see if he was obliging by the speed limit," wrote the tennis star. "Then I remembered that horrible video of the woman in the car when a cop shot her boyfriend."
Williams was likely referencing the fiancee of Philando Castile, who streamed the aftermath of her fiance's fatal shooting in July by police
live on Facebook.
"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.'"
The 22-time grand slam champion is the latest of a series of high profile sports stars to protest police violence against black Americans.
Basketball's LeBron James said Monday
that the thought of his son getting pulled over driving when he's old enough was "scary," adding "I'm not that confident... that my son is going to return home."
San Francisco 49ers quarterback Colin Kaepernick sparked outrage but also support
in August when he refused to stand for the US national anthem in a pre-game ceremony, a statement against black "oppression".
"I am a total believer that not "everyone" is bad," voiced Williams in a post
that has been shared over 16,000 times. "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."
The 35-year-old also wrote about her own role and future.
"I then wondered then 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?"
Williams ended her post referencing civil rights activist Martin Luther King: "As Dr. Martin Luther King said, 'There comes a time when silence is betrayal. I won't be silent."
Her statement comes after protests in Charlotte
, North Carolina last week following the fatal police shooting of 43-year-old Keith Lamont Scott.