"Performing is very fleeting, it's very intangible, it's very momentary. And it's wonderful. But it's not lasting, and if you can do things that last, that each generation can build upon, then that's when you're cooking."

These are the words of Billie Jean King, delivered with a serenity - and the trace of a smile - which contrasted sharply with the stomping petulance she once upon a time displayed on the tennis court.

It was a face, a very human one, that was never allowed to surface in her great playing days, those days of Wimbledon when the crowds turned on her because she was "too professional", or, as we were reminded in a vox-pop from the late 1960s, because "she charges about the court like a man".

Really good sports documentaries, the type that scratch more than skin deep, are few and far between. So, it is to RTE's credit that they brought us Billie Jean King: Rogue Champion - previously screened on BBC - on Tuesday night. It demonstrated superbly how we treat sporting icons as our property.

The irony was that King, the greatest women's tennis player of all time, was iconoclastic.

As the archive footage changed from scratchy black and white images into glorious technicolour, we became aware that King was primarily responsible for dragging women's tennis from a sideshow into a multi-million dollar industry. To do so, she had to knock down walls and overcome prejudice. She had to endure isolation, and she had to show tremendous mental strength. As one contributor to the documentary stated, with just the hint of a grin, "a lot of people thought of her as a strident bitch".

Billie Jean King, winner of 39 Grand Slam titles and 20 at Wimbledon, wasn't born with a silver spoon in her mouth. She learnt how to play on the public courts of Houghton Park in Long Beach, California. It's a place of oil wells and aviation factories. He dad was a fireman, her mother an Avon lady. As her biographer, Frank Deford, said in the documentary of King: "You couldn't ask for a more typical American little girl . . . she was supposed to grow up to be a typical American south Californian housewife."

She didn't. When the tennis bug struck as an 11-year-old, her parents "wore out" three cars transporting her to different local tournaments. Instead of playing tennis in public parks, she progressed to playing tournaments in swish country clubs. Even then, as a young teenager, she didn't fit in.

"They were elitist. I didn't feel good, didn't feel right," she recalled.

Back then, tennis was a purely amateur sport. But King got to the stage where she was sucked into the world of secret payments, of brown envelopes being slipped under the table so that she would play in tournaments. "It started killing me, it was disgraceful," said King of the shamateurism. We saw archive footage that showed how the Wimbledon crowds disliked her. How she would refuse to sign autographs because she had to practice. "If I sign it for you, I'll have to do it for everyone," she told one hunter. Her voice was cold and stern.

But the amateur ethos of Wimbledon, and the approach of the English to tennis, was at odds with King's philosophy. To her, it wasn't a social game. At the time, she mimicked how they would enjoy a "good fight (on the tennis court)" or someone who had been "a jolly good sport". To King, it was more serious: "If you lose, you're a loser and that's it . . . I am not going to walk on the court unless I can win."

Such an aggressive streak enabled King to change tennis, to be the prime mover and shaker. However, as her contemporary, Rosie Casals, told us, when King led the move from amateurism to professionalism, she was accused by some of her fellow players of "prostituting yourself" by playing for money. The likes of Virginia Wade, Yvonne Goolagong and Chris Evert, at first, refused to join King's professional circuit.

But King broke through the hypocrisy of shamateurism and, within four months of forming her breakaway tour in 1968, Wimbledon welcomed in the professional era. Those women players who this week will be chasing barrows full of money at Wimbledon owe a great debt to King, a player whose first singles title at the All England club won her a £45 gift voucher for Harrods.

Even when King fashioned the new professional era for women, the struggle went on. Women's tennis was perceived as a sideshow to the men's event. Prizemoney was little more than a 10th of what the men got. She acted like a union shop steward and changed things again. The women broke away. In footage of King from that time, she informed us that all she was trying to do was to "create equal opportunity and to share, not dominate".

"People looked at her as if she was crazy, that she belonged in a looney bin," remarked Evert. "But she was the one willing to stick her nose up at the establishment, the one who had vision."

Billie Jean King was a feminist. When she heard that Margaret Court had lost to 55-year-old Bobby Riggs in what was called the "Mother's Day Massacre", her instinctive reaction was that she would now have to take him on. King was getting onto a plane somewhere when she heard the result of that particular match. "I just got lockjaw when I heard the result," she said. She knew, too, that she would be called upon to "defend the sisterhood".

So it was that more people remember Billie Jean King for a $100,000 winner-takes-all tennis match against a male chauvinist than for any of her real titles. The match gripped the American public. At the time, 50 million viewers watched the encounter on ABC. It was a match that divided households. Even her fellow-professionals thought she wouldn't win.

"I didn't think she had the weapons, the guns to overpower him," said Evert of the match. So it was that Riggs, in a psychedelic yellow top with the words "Sugar Daddy" on the back, took on King in a Battle of the Sexes match. King, who had become a cause celebre for the women's movement, won. Elton John wrote a song in her honour and had a number one hit.

When King - who had been married for two decades - retired, she was "outed" as a lesbian. "It was the single most difficult struggle in my life," she said. Commercial contracts were lost. She got divorced. She had to go back to playing tennis to pay the lawyers.

These days, she works as a television analyst and coaches kids in Chicago. Instead of the public courts that bred aggressive champions like King, the kids attend coaching academies. "They all look the same," said Martina Navratilova.

Billie Jean King could never be accused of looking or acting the same as anyone else. Her giant, horn-rimmed glasses and sometimes straight, sometimes permed hair didn't look as if they belonged to a sporting icon. But she did more than win championships. She changed the game. All these years later, her legacy is cooking.