Serena Williams vowed on Tuesday night that she “won’t be silent” about the killing of black men by police officers in the United States. Williams wrote on Facebook that she was inspired to speak out after asking her black 18-year-old nephew to drive her to a meeting. Their car passed a police officer and Williams “quickly checked to see if he was obliging by the speed limit” so as to attracting attention. Williams recalled “that horrible video of the woman in the car when a cop shot her boyfriend” — a reference to when Philando Castile was shot by a Minnesota police officer in July — and said she worried about similar fates for members of her family. “I even regretted not driving myself,” she wrote. “I would never forgive myself if something happened to my nephew. He’s so innocent. So were all ‘the others’.” Williams also quoted Martin Luther King Jnr, saying: “There comes a time when silence is betrayal,” before ending the post with “I Won’t Be Silent.” Williams joins a high-profile group of athletes speaking out for social justice. San Francisco 49ers quarterback Colin Kaepernick began kneeling during the US national anthem in the pre-season, which has drawn widespread criticism from past players and politicians.