Without a long line of female players to look up to, Casals developed her imagination through sheer determination. “It was an instinctive thing I felt, that I was good,” she describes. Competing in junior tennis, Casals defied skeptics of her 5’2” height and enrolled in tournaments with older players. She earned a “rebel” characterization for the actions she took to push the game forward. “The rebel part was always about wanting more,” explains Casals. “As a woman, or as a girl, you're always fighting for more. I had the fight in me to be better, to win. When we had to make decisions for the future of women’s tennis, the fight continued.”

In 1970, Casals wrote a piece in World Tennis Magazine predicting the end of the women’s game due to the unfair and unsustainable discrepancy in prize money. It was published at a time when women “played everything to pay for everything,” she says, from singles to doubles to mixed doubles. “We were making $3,000 a week, which is nothing if you're paying for your travel, dinners, and hotels.” In the early days, Casals’ celebrated doubles and mixed doubles careers were not just a complement to her singles matches; they were a matter of survival. “I wrote that article based on the observation that we needed to do something,” remembers Casals. “We were not going to get our piece of the pie. At the time, women were supposed to get married, have kids, and stay home. There were no scholarships for us. The men didn’t like that we were competing for the same prize money. We didn’t have much to look forward to.”

Early members of the Original 9 approached promoter and World Tennis Magazine founder Gladys Heldman, who helped recruit Joe Cullman of Philip Morris to put on the Virginia Slims Circuit, comprising eight professional tournaments. Heldman put forward $5,000 of her own money for the group’s seven Americans and two Australians, who initially competed for a $7,500 purse.

In direct defiance of the United States Lawn Tennis Association (USLTA), the Original 9 signed symbolic $1 contracts in Houston, Texas, on September 23, 1970. All at varying points in their careers, they risked losing future income, in addition to national rankings and entry into Grand Slam tournaments. Their unity was their superpower. “We got nine women to go against the old establishment, to make the same decision, the right decision,” says Casals. “I'm proud to be part of the Original 9 women who went out on a limb and took that chance.”

The tournament grew from 9 players to 40 players and 19 events across the U.S. in 1971, with a new purse of $309,100. Players in Europe, also frustrated by a lack of equal prize money, traveled to the U.S. to compete. “At that time, European tennis hardly existed,” remembers Casals. “They came over, and we had a great circuit that was very successful. The promoters made money, and the players made money. We put women's tennis on the map.”

While they initially defined success by equal pay, the Original 9 were deliberate about ensuring a sustainable future for women’s tennis players. Their unfiltered talent finally had the backing it deserved, and the effects reverberated globally in the years and decades to come. “Women's tennis was very hot in ’73, ’74, and ’75,” explains Casals. “We were filling all the stadiums.”

Casals continued to rack up wins during this era, including the Family Circle Cup in 1973, where she won $30,000, the biggest prize money in the history of women’s sports at the time. It was also the first women’s tennis tournament ever to be broadcast on network television. Her acrobatic talent was matched by her outfits, made up of colorful headbands, velvet, and rhinestones. A hint of purple on her Ted Tinling dress, worn during Wimbledon in 1972, made headlines when organizers asked her to change into the tournament’s strict, all-white dress code.

“Ted designed my outfits based on how I played,” recalls Casals. “He felt that I was flare and theatrics, and he always made me flashy clothes. I was a shotmaker, so it was fun being dressed like I was entertaining. Most of the times when we played at night, in those big arenas, they'd have a nice spotlight. You felt like you were on Broadway.”

The 1973 founding of the Women’s Tennis Association (WTA) solidified the efforts of the Original 9, just a year after the passing of Title IX anti-discrimination legislation (created to give women equal opportunity in institutional sports, though today, it extends to all gender identities and educational systems, with built-in accountability measures). The WTA united all of women’s professional tennis in one tour, with a mission of ensuring a better future for all women who play the game. Remembering the crucial meeting of the WTA’s soon-to-be first members in London, Casals says: “I think most of us — players from Europe, Sweden, Denmark, South America, and Russia — saw the writing on the wall and agreed that the only way to get things done was to be united.” Soon after, the US Open became the first tournament to offer equal prize money to men and women.

The WTA’s first president, Billie Jean King, was not only Casals’ doubles partner and Original 9 colleague; the two were also singles competitors and friends. Casals always admired King, who became the first woman to make six figures in prize money in 1971. “Billie Jean King was the leader; she was number one,” says Casals. “I looked up to her. She was five years older. She asked me to play doubles with her at Wimbledon when I was 17, and I had never been out of the country. In singles, she sometimes played her best tennis against me, and I had to hear about it all the time. We had a great friendship, a great doubles partnership, and a great rivalry.” Casals and King still meet up to play tennis to this day.