Whether long-time American legend Serena Williams, Japanese hotshot Naomi Osaka or Canadian sensation Bianca Andreesu captures the title at a tennis Grand Slam tournament these days, one thing’s for sure: The women’s players will get the same prize money as the men’s winner.

But that wasn’t always the case. The Open Era of tennis that we know today started in 1968, allowing professionals and amateurs to compete together for prize money. The gap between the two genders was blatant from the start, with the 1970 men’s winner Ilie Nastase earning $3,500 while Billie Jean King got $600.

King—who went on to win a total of 39 Grand Slam titles including 12 singles crowns—was not about to sit silent, taking home more than five times less than her male counterpart, so she raised the flag loudly, famously saying: “Everyone thinks women should be thrilled when we get crumbs, and I want women to have the cake, the icing, and the cherry on top too.”

When King was 12, she played in a 1955 tournament at the Los Angeles Tennis Club and was ready to step into a group photo of junior players, until she was asked to step out of the frame. The reason: She was wearing shorts instead of the traditional tennis skirts the other girls were wearing. The early moment was eye-opening for the rising star, as she got her first taste of gender disparity in the sport she loved.

By the time she captured her first Wimbledon win in 1966 as an amateur, she also soared to the top of the rankings, becoming the No. 1 women’s player. Despite that status, she was still a Los Angeles State College student scraping by on $100 a week as a playground instructor.

Changes were afoot as the Open Era started, but even when King won her first Wimbledon, she got £750 (about $1,064) as opposed to the men’s winner Rod Laver, who scored £2,000 (about $2,840).

“I didn't have any idea we were going to get different prize money,” she said on American Masters. “I thought it was totally unfair.”

The gap was disheartening, but even more so were the waning opportunities for women to play. “From ‘68 to ’70, we just had less places to compete,” King told Tennis Channel of tournaments being dropped and women getting eight times less pay than men. “The writing was on the wall. If you look at old quotes in the old days, around the late '60s and '70s, you'll see that the men were telling us we should quit and go take care of our husbands.”

But the female players weren’t about to throw in their racquets. Instead, they made a racket of a different kind. They started a tournament of their own in September 1970 with the help of World Tennis Magazine publisher Gladys Heldman.

Along with King, tennis stars Peaches Bartkowicz, Rosie Casals, Judy Tegart Dalton, Julie Heldman, Kerry Melville, Kristy Pigeon, Nancy Richey and Valerie Ziegenfuss became known as the Original 9, famously signing on to the Virginia Slims Invitational in Houston for $1 each—and posing for an iconic photo where they each held up a single dollar bill. That single digit deal allowed them to become “contract pros.”

“We weren't sure about our destiny but we knew it was in our hands for the first time,” King said.

While the move was bold, the pay discrepancies continued. Though King was celebrated in 1971 as the first woman athlete of any kind to make more than $100,000—even wearing a crown with the six figures on it and receiving a call from President Richard Nixon—but it still paled to what the men were making for the same line of work. By the 1972 US Open, King was taking home $15,000 less than the men’s winner.

Ahead of Wimbledon in 1973, she barricaded 63 players in London’s Gloucester Hotel, and officially formed the Women’s Tennis Association. Later that year, she threatened to boycott the US Open unless the women’s prizes were equal to the men’s.

It worked. Thanks to a grant from Ban deodorant (appropriate since King had once said the disparity “stinks"), both the men’s and women’s singles winners would take home $25,000.

Even with that landmark Slam—which Australian Margaret Court won—King wasn’t done proving that women deserved to be on equal ground. On September 20, 1973, she accepted a taunt by Bobby Riggs, a male tennis player who was famously chauvinistic, once saying, “Women belong in the bedroom and kitchen, in that order.” He had long been trying to challenge women’s players, beating Court that May in a straight-set victory that became known as the Mother’s Day Massacre.

Though King had turned him down before, she agreed to a $100,000 march to which Riggs said, “I'll tell you why I'll win. She's a woman and they don't have the emotional stability. She'll choke.”

While women’s tennis, even today, calls the winner based on winning two out of three sets, the rules for what became dubbed “Battle of the Sexes” followed the men’s rules of three out of five.

As they went head to head, King showed who was the true king of the courts, dominating in a straight-set win over Riggs, 6-4, 6-3, 6-3. With that, she didn’t just prove equality, she also took home the paycheck she long deserved.

Even with those landmark moments more than four decades ago, the road ahead was long. After the US Open’s equal pay move in 1973, the other three Grand Slam tournaments were slow to follow. The Australian Open joined in 1984 but didn’t again from 1996 to 2000. And it wasn’t until 2006 that the French Open (also known as Roland Garros) followed and finally Wimbledon in 2007.

“What started as a few women and a dollar has grown to thousands, living the dream, our dream,” King said of the long path. “We were athletes who wanted to compete—and along the way we made history, determined to win, not just for ourselves, but for women everywhere.”