Tennis star Serena Williams "won't be silent" on police violence in the USA

Tennis star Serena Williams has spoken out about police violence in the United States by insisting that she "won't be silent".

In a Facebook post, the 22-time Grand Slam champion admitted to feeling anxious when she eyed a policeman while being driven to a meeting by her 18-year-old nephew.

The former world number one appeared to refer to the shooting of Philando Castile, whose girlfriend live-streamed an incident with police after her partner was repeatedly shot by a white officer during a routine traffic stop in Minnesota in July.

In the post, Williams wrote: "In the distance I saw cop on the side of the road. I quickly checked to see if he was obliging by the speed limit. Than I remembered that horrible video of the woman in the car when a cop shot her boyfriend. 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'.

"I am a total believer that not 'everyone' is bad 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.

"I than wondered than 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? As Dr. Martin Luther King said: 'There comes a time when silence is betrayal'. I won't be silent."

Williams is the latest American sports star to speak out about the police brutality. San Francisco 49ers quarterback Colin Kaepernick held a protest by refusing to stand for the national anthem.