When Billie Jean King won the Italian Open in 1970, her prize was $600. Her male counterpart, Ilie Nastase, was awarded $3,500, nearly six times that. Two years later, King and Nastase both won the U.S. Open. This time, he received $15,000 more than she did.
King said she decided at age 12 that she wanted to fight for equal rights for girls and women. And she used the tennis court as her platform.
"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," said King at the Miami Open tennis tournament in March. The athlete has won 12 grand slams.
After King threatened to boycott the U.S. Open in 1973 over pay disparity, it became the first major tournament to award equal prize money to its male and female champions. Wimbledon, the oldest tennis tournament in the sport's history, was the last Grand Slam event to follow suit in 2007, after Venus Williams led the charge for equal pay.
Today, thanks to the advocacy of King and other female players', tennis is one of the few global sports that pays its men and women the same amount in major tournaments.
But scrutiny over pay in tennis was reignited, again, last month.
In March, Indian Wells CEO Raymond Moore said the Women's Tennis Association, co-founded by King in 1970, was a "lucky" organization that rode "on the coattails" of men's tennis. Serena Williams, the world's top-ranked women's tennis player, called the remarks "very, very, very inaccurate." Moore resigned two days later.
Novak Djokovic, the highest-ranked men's tennis player, then asked if women deserved equal pay, adding that the "men's tennis world … should fight for more." The male and female Indian Wells winners, Djokovic and Victoria Azarenka, both received $1.03 million for their tournament victories. Djokovic has since walked back the comment.
David Berri, co-author of "Stumbling on Win" and an economics professor at Southern Utah University, said the men who are making controversial statements over equal pay have no idea the race is biased in their favor when men's sports get to play on bigger networks with better promotion.
"There's a blindness here where people don't get the idea they're starting in different positions," Berri said.
All four Grand Slam events may have propelled tennis ahead of other sports in equal pay, but the sport still grapples with other factors that prove more beneficial to male athletes.
Outside those major tournaments, the pay disparity becomes much more pronounced at other large events. The Western & Southern Open, which happens weeks before the U.S. Open, paid Roger Federer $731,000 for winning the men's tournament, while Serena Williams received $495,000 for winning the women's tournament, The New York Times reported.
On Forbes's list of the highest-paid athletes, Federer also ranked first in endorsement dollars with an estimated $58 million in 2015, far surpassing Maria Sharapova's $23 million or Serena Williams' $13 million.
But gender equality in sports goes beyond the money, Berri said.
Leadership among the four major tournaments is mostly white and male. Katrina Adams, CEO of the U.S. Open, is the only woman who bucks the trend, Berri said.
Also, as FiveThirtyEight has pointed out, there can be an imbalance over who gets to play on the main courts of major tournaments, where journalists, fans and cameras train their focus. At the Wimbledon last year, organizers allocated only 38 percent of scheduled spots for women's matches, giving them less opportunity to foster and build a fan base.
Despite these inequalities, "If you look at other sports, such as soccer or basketball, it is the case that there's more wage equality in tennis than other sports," said Berri.
Just last month, the U.S. women's national soccer team filed a complaint with the Equal Employment Opportunity Commission for being paid four times less than their male counterparts in the 2015 World Cup.
Berri said that difference of promotion can also seen with the expansive production value afforded to the NCAA men's basketball tournament, an example of unequal coverage. The men's tournament had multiple camera angles covering the event. Meanwhile, during the women's tournament, broadcasters were remote — not present in the arena, Berri said.
Shortly after Moore and Djokovic made their comments about women's tennis, a news conference with King and fellow tennis great Chris Evert was organized at the Miami Open to discuss such resistance from male players over equal pay.
After tweeting she was "disappointed" in Moore's comments, King said at the Miami Open that she wanted to move past the controversy because "no one's perfect."
In her book, "Pressure is a Privilege," King talked similarly about her opponent in 1973's winner-take-all "Battle of the Sexes" match between her, then 29, and 55-year-old former champion Bobby Riggs.
With second-wave feminism as its backdrop, King said media played the gender angle to the hilt. The telecast began with the "glib tune" of "Anything You Can Do, I Can Do Better" and an ABC announcer spent more time ruminating about her looks instead of her accomplishments, King said.
Riggs arrived in a rickshaw pulled by his "bosom buddies," while, for her part, King agreed to being carried out by four, muscular track and field men like Cleopatra. In the ramp-up to the event, Riggs had said a series of offensive comments, prompting King to present him with his own "chauvinist pig," a live piglet adorned with a pink bow.
In front of 30,000 people in Houston's Astrodome and an estimated 48 million Americans that tuned into the match on their televisions, King would beat Riggs in a 6-4, 6-3 and 6-3 victory.
Despite Riggs' rabble-rousing, one statement from the male player stood out from the rest: "I really underestimated you," King remembered him saying after he lost the match.
"I think the press was a bit surprised to hear me say that I was not playing the game to prove that women could beat men," King said in "Privilege." "I was playing to prove that men and women had the same entertainment value, which is why we should be paid equally."
Back at the Miami Open, King said although female tennis players made $14 dollars a day back then, it seemed that women had fewer and fewer tournaments to play. She and her colleagues helped found the WTA and started a professional women's tour, King said. Their goal was that "any girl in the world, if she's good enough, would have a place to play, be recognized for her accomplishments — not just her looks — and make a living."
Decades after "Battle of the Sexes" and in light of the pay disparity changes that have been made to the sport, King said "tennis players are the lucky ones."