Serena Williams is a legend on the tennis court and a megastar off it, and with over seven million followers on Twitter she sits near the top of the list of most followed athletes on social media.

Now, Serena has vowed to use that reach to speak up about race relations and police-involved incidents in the United States, particularly on the allegations of increased police violence toward unarmed African Americans.

Serena took to social media to post a heartfelt and powerful message, candidly expressing her fears for the safety of her 18-year-old nephew.

“I had to take a look at me. What about my nephews? What if I have a son and what about my daughters?” Serena said in a lengthy post to her Facebook page. “As Dr. Martin Luther King said, ‘There comes a time when silence is betrayal.’ I Won’t Be Silent.”

It’s not the first time Serena has commented on the issue, previously speaking out about police brutality after her record-tying Wimbledon victory.

“I feel anyone in my color in particular is of concern. I do have nephews that I’m thinking, ‘do I have to call them and tell them, ‘Don’t go outside. If you get in your car, it might be the last time I see you?'” she told reporters back in July.

“I don’t think that the answer is to continue to shoot our young black men in the United States. It’s just unfortunate. No one deserves to lose their life, doesn’t matter what color they are, where they’re from. We’re all human.”