Here's why Serena Williams didn't spend a dime of her first million-dollar check

Despite playing a lighter than normal schedule this year following the birth of her daughter, tennis star Serena Williams remains the highest-paid female athlete in the world, according to Forbes. With the U.S. Open officially starting on Monday, she stands to earn another $3.2 million if she's able to clinch another grand slam title.

But though her career has been lucrative, Williams doesn't blow through cash. In fact, after earning her first million, Williams didn't touch any of the money — she deposited it directly in 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 recalled in a 2017 interview with Uninterrupted, a media company owned by LeBron James and Maverick Carter.

Williams said 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 said. "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 causes of gender equality and closing the pay gap.

After her semifinals victory at Wimbledon in 2016, 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 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 wrote.

She went on to say that she embraces what others views as "disadvantages," including her race and gender, and turns 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 squirrel cash away. 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."

This is an updated version of a previously published article .

Don't miss: 9 billionaires who drive cheap Hondas, Toyotas and Chevrolets

Like this story? Subscribe to CNBC Make It on YouTube!