Forty years after her famous Battle of the Sexes match against Bobby Riggs, former tennis star Billie Jean King recalls her fight against gender inequality in tennis as well as her work for gay rights.

The night before the most-watched match of Billie Jean King’s career — which turned out to be the most-watched match in tennis history — she called her brother, Randy. It was September 1973. Not only was this match important, but a loss would mean irreparable damage to her reputation, to the women’s tour that had started three years earlier, to the Women’s Tennis Association that had started three months earlier, to the legislation designed to end sexual discrimination in schools and colleges — and to the burgeoning, brilliant women’s movement as a whole. Randy asked if she was going to win. “Go ahead and bet the house,” she replied.

King is 69 now, and we are here to talk about that defining match, dubbed The Battle of the Sexes, which has just become the subject of an excellent documentary of the same name. The film brings out all the tensions and colour of the time, prompting reflection on the inequalities that still persist.

The idea for the match came from King’s opponent, Bobby Riggs, who was 55. Feminism was headline news, and Riggs sniffed an opportunity. He started approaching the top women’s players, dangling a big pay day as an inducement. King was the one he really wanted; he called her “the sex leader of the revolutionary pack”. King had been playing on the national circuit since 1959, and had first played internationally, at Wimbledon, aged 17, in 1961. She was one of the players who pushed for the game to be “open” — for professionals and amateurs to be allowed to play in all competitions, together. But then she realised that women players were to be paid much less than the men. And in many competitions, women weren’t even given the chance to play.

King knew that some of the male players were planning to form a professional organisation. “You are going to include us, aren’t you?" she asked. And they said: ‘Absolutely not.’

King and some of the other women players contacted Gladys Heldman, the influential founder of World Tennis magazine, and explained their situation; they decided to start a women’s tour. This meant boycotting the tennis establishment. The US Lawn Tennis Association, as it was then, told them they would be suspended if they went ahead, and wouldn’t be able to play in competitions including Wimbledon and the U.S. Open. They decided to do it anyway. On September 23, 1970, nine women players — seven American, two Australian — were pictured holding the symbolic $1 they were being paid to join the tour.

Next on the agenda for King was speaking to politicians about the legislation that would become Title IX, which passed into law in 1972, and stated that “no person in the United States shall, on the basis of sex, be excluded from participation in, be denied the benefits of, or be subjected to discrimination under any education programme or activity receiving federal financial assistance”. This changed the lives of countless female athletes, who could suddenly receive the scholarships that King herself had been denied. Led by King, the women went on to found the Women’s Tennis Association — a body that gave them one voice — in a secret meeting in London, during Wimbledon, in June 1973.

Everything was going well, except for the continued nag of Riggs. On Mother’s Day, 1973, an event forced King’s hand. Margaret Court, an Australian player who was King’s greatest rival at the time, and who had kept herself away from the essential ferment of the feminist movement — “I am not carrying the banner for Women’s Lib,” she declared — agreed to Riggs’s proposition. At the start of the match, he presented her with a bunch of roses. She curtsied, to the horror of many women viewers. And then she allowed his style and braggadocio to rattle her. It took 57 minutes for Riggs to beat her 6-2, 6-1, in a match that became known as the Mother’s Day Massacre.

King knew if she left Riggs’s victory hanging, critics would use it to ridicule the women’s game for ever after. In some ways, the situation must have seemed like destiny. She grew up in Long Beach, California, the daughter of a fire-fighter, and was 11, she says, “when I decided I wanted to be number one. That was the second time that I played tennis. By 12 I had an epiphany about trying to change tennis. Because I saw that everyone who plays wears white socks, white shoes, a white tennis dress or shorts, and they’re all white. My question to myself, as a 12-year-old, was: where is everybody else? There are no people of colour. Something’s not right.”

King’s understanding of inequality deepened at California State University, Los Angeles. There, at 18, she met a man called Larry King (not the broadcaster), who was a year younger than her; they married in 1965. He had been listening to her frustrations, and one day said to her: “‘You’re a second-class citizen.’ And I go: ‘What do you mean?’ And he said: ‘Because you’re a girl.’ And that day just solidified all these feelings I had bubbling up in me, all the things I’d been thinking and talking about. He just made it simple, you know”. After Court’s defeat, she agreed to play Riggs, and a great hoopla built up around the match.

The attention represented another threat for King. In the late 1960s she had started to realise that she had feelings for women, and in 1969 asked Larry for a divorce. He refused on the basis that he still loved her. The lives of the pair were intertwined — they owned tennis tournaments together, began a women’s sports magazine together — and she loved him too. But she had also begun a relationship with her personal assistant, Marilyn Barnett, and was in terror of this being revealed. An onslaught of press attention was the last thing she needed.

The match was played on September 20, 1973; men wore their male chauvinist pig T-shirts, women arrived with banners supporting Billie Jean. The worldwide TV audience was 100m. She knew that whether she won or lost, she would be remembered for this match for the rest of her life. She won, of course, in straight sets. It was one of the great spectacles of the women’s liberation movement, featuring a woman who had always tried to argue much more subtly for equality. “You can see with my interviews back in those days, I was walking this tightrope to not alienate.”

She can speak more freely these days. King was outed in 1981 when Barnett brought a palimony case against her; as one of the first openly gay athletes she lost all her endorsements overnight. She and Larry remained married until 1987, and friends after that — she is godmother to his children.

In the years since, she has worked hard for gay rights, which she describes as the civil rights issue of the 21st century, “the first half anyway”, and for women’s rights. It is in large part her legacy that tennis is now the only sport in which men and women receive equal pay. “It’s not about the money, it’s about the message we send. We are sending the equality message out that this is the right thing to do. Yes, the men are better than us in some ways. Yes, we’re better in some ways. It doesn’t matter. Don’t you want to share in this world? I do.”

“Everyone’s an influencer,” she says. “It’s not only what you do on the court — do you realise how you can affect people in your village, in your town, in your country, in this world? You have a platform that very few people will ever have ... You hit a tennis ball with a racquet over a little net, and just think what you can do with that, beyond trying to win Wimbledon.” So speaks one who knows.

© Guardian Newspapers Ltd. 2013