But that doesn't mean she blows through cash. As Business Insider first pointed out, after earning her first million, Williams didn't touch any of the money — she deposited it directly into the bank and walked away.
"I remember, I went through the drive-through to deposit my check and they were like, 'I think you need to come in for this,'" she recalls in an interview with Uninterrupted, a media company owned by LeBron James and Maverick Carter.
Williams says that, for her, tennis has never been about getting paid. She's always played for the love of the game. In fact, she used to forget to collect her paychecks at all.
"When I first turned pro, you had to go pick up your check," she says. "I never, never picked it up, so at the end of the year, the tournament directors would literally hand me the check because I would never go get it."
That's not to say that Williams doesn't believe in getting what she's rightfully owed. Williams and her sister Venus have promoted the larger cause of gender equality and closing the pay gap.
After her semifinals victory at Wimbledon last year, Williams corrected a reporter who asked what it feels like to be one of the greatest female athletes of all time, saying "I prefer the word 'one of the greatest athletes of all time,'" Sports Illustrated reports. (As Andy Murray just demonstrated when he corrected an interviewer who seemed to have overlooked Williams' achievements, some other tennis champs also have her back.)
And in an open letter written for Porter Magazine's Incredible Women of 2016 issue and published in The Guardian, Williams took aim at the gender pay gap both in tennis and society at large.
"Too often women are not supported enough or are discouraged from choosing their path," she writes.
She goes on to say that she embraces what others views as "disadvantages," including her race and gender, and channels them into "fuel for my success."
"When the subject of equal pay comes up, it frustrates me because I know firsthand that I, like you, have done the same work and made the same sacrifices as our male counterparts. I would never want my daughter to be paid less than my son for the same work. Nor would you."
As far as earnings go, Williams isn't the only celebrity to stash cash away instead of spending it. To this day, comedian Jay Leno hasn't touched a dime of his "The Tonight Show" money.
"When I was younger, I would always save the money I made working at the car dealership and I would spend the money I made as a comedian," Leno told CNBC. "When I started to get a bit famous, the money I was making as a comedian was way more than the money I was making at the car dealership, so I would bank that and spend the car dealership money."
Even after he was earning up to $30 million a year as the host of the late night program, Leno continued to pursue comedy gigs on the side so he'd never have to rely on his primary income.
Leno's strategy gives him peace of mind.
"So many people get to be the age I'm at now and they've got nothing because they just blew it all," he said. "It sounds ridiculous, but if everything ends tomorrow, I know I'll be fine."
Like this story? Like CNBC Make It on Facebook.