(CNN) -- American tennis star Serena Williams has slammed the gender pay gap, issuing a passionate call for black women to "be fearless" and demand equality.

In a highly-charged personal essay, published by Fortune Magazine, the 23-time grand slam champion writes that for every dollar a man earns, black women make just 63 cents.

"The cycles of poverty, discrimination, and sexism are much, much harder to break than the record for Grand Slam titles," said Williams, decrying statistics showing "women of color have to work on average eight months longer to earn the same as their male counterparts do in one year."

"For every black woman that rises through the ranks to a position of power, there are too many others who are still struggling."

Though Williams was recently named the world's highest-paid female athlete by Forbes -- with annual earnings exceeding $25 million -- the American is only 51st on the overall list, and far behind ATP Tour contemporaries Roger Federer, Novak Djkovic, Kei Nishikori, Rafa Nadal and Andy Murray.

She has previously been forced to defend how much female players receive at Grand Slam tournaments, despite Wimbledon only instituting equal pay for singles winners in 2007.

"In every stage of my life, I've had to learn to stand up for myself and speak out," said Williams, who is currently taking time out of the game to give birth to her first child.

"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."

Just this month, former world No. 1 Ilie Natase was banned from sport by the International Tennis Federation until 2021 for "racially insensitive" remarks made in April about Williams' unborn child.

Seven-time grand slam champion John McEnroe, meanwhile, has been widely condemned for comments that Williams would be "like 700" in the men's world rankings -- later clarifying but refusing to retract his remarks.

In every instance, Williams has refused to be silent -- telling McEnroe to "respect my privacy" and drawing on a Maya Angelou poem in her condemnation of Natase.

Last year, she addressed inequality in the game in an open letter, following comments from Indian Wells CEO Raymond Moore that the female players "ride on the coattails" of the ATP Tour game and should "get down on their knees every night" to thank male players.

"I don't think any woman should be down on their knees thanking anybody like that," Williams told reporters. "I don't think that is a very accurate statement."

Moore later apologized for the remarks, but stepped down from his role as tournament director.

This latest essay speaks to a far wider disparity - one she argues is as prevalent "in inner cities as it is in Silicon Valley."

"Today isn't about me," writes Williams. "It's about the other 24 million black women in America. If I never picked up a tennis racket, I would be one of them; that is never lost on me."

"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."