The record-breaking tennis star and world's highest paid female athlete -- who has spoken candidly about sexism and racism -- wrote a nearly 1,000-word essay in Fortune magazine to mark Black Women's Equal Pay Day.

For every dollar made by a man, black women make 63 cents in the United States. Black women earn 17 percent less than white women, and black women with graduate degrees get paid less at every level, wrote Williams.

The 23-time Grand Slam singles champion quoted data showing that only 44 percent of white men recognize the pay gap as an issue.

"I have been treated unfairly, I've been disrespected by my male colleagues and -- in the most painful times -- I've been the subject of racist remarks on and off the tennis court," wrote the 35-year-old.

"The cycles of poverty, discrimination, and sexism are much, much harder to break than the record for Grand Slam titles," she added, calling for legislation and action to change the status quo.

Williams, who announced her engagement to Reddit co-founder Alexis Ohanian last December, is pregnant with her first child. She plans to return to competitive tennis next year.

"Black women: Be fearless. Speak out for equal pay. Every time you do, you're making it a little easier for a woman behind you," she said.

"Most of all, know that you're worth it. It can take a long time to realize that. It took me a long time to realize it. But we are all worth it. I've long said, 'You have to believe in yourself when no one else does.'"

Forbes ranks Williams as the highest earning female athlete with a pay check of $27 million -- but only 51st on an overall list of highest paid athletes. Forbes ranks Roger Federer as the world's highest paid tennis star at $64 million.

"Black women are the cornerstone of our communities, they are phenomenal, and they deserve equal pay," Williams tweeted Monday with a picture of herself wearing a T-shirt saying "Phenomenal Woman."