Tennis superstar Serena Williams on Tuesday voiced her fears about the United States' race problems and spoke out against police killings of African Americans. In a Facebook post, she promised to be more vocal on the subject and quoted Martin Luther King by saying, “'There comes a time when silence is betrayal’. I won’t be silent.”

Williams said that earlier in the day, she had seen a police vehicle pull up next to their car while her nephew, who is black, was driving with her in the passenger’s seat. She said this brought back memories of Philando Castile, who was shot dead in front of his girlfriend after being pulled over by the police. The Castile incident came to light after his girlfriend made a Facebook Live video in the aftermath of the incident. Williams wrote, “I remembered that horrible video of the woman in the car when a cop shot her boyfriend. I would never forgive myself if something happened to my nephew. He’s so innocent. So were all ‘the others’.”

Williams is not the first sportsperson to have voiced their protest against racism in policing. On Monday, basketball champion LeBron James said he feared for the lives of his children, reported The Guardian. In August, footballer Colin Kaepernick refused to stand up during the national anthem to mark his protest against “a country that oppresses black people and people of color”. Later, many more players of the National Football League joined in, reported The Guardian.

Williams’ post assumes significance in the backdrop of the killing of Keith Lamont Scott in Charlotte, North Carolina. In July, three police officers were killed in Baton Rouge, Louisiana, after a resident from Missouri opened fire at them to protest the death of an African-American at the hands of the local police. According to BBC, around 214 black people have been killed by the United States Police this year.