Billie Jean King remembers it well. It was 5 July 1968. The American had just won her third straight Wimbledon title, her second grand slam title of that year and her fifth overall at the time. But this one meant that little bit more, the first Wimbledon of the Open era, as tennis finally welcomed back the professionals into the sport, ending the era of “shamateurism” when amateurs were paid under the table. Overjoyed after her win over the Australian Judy Tegart, King collected the trophy from Princess Marina of Kent and then, for the first time, pocketed a cheque as well. That was when things went wrong.

“I knew we were starting to go in the right direction because it was professional and then the prize money was there,” King says. “Then I had another shock, though, at the finals. Because I didn’t really look at the money – I just knew we had money – I got £750 and Rod Laver got £2,000 and I was like: ‘Oh God, here we go again ... argh, there’s another battle, we’ve got pro tennis but now the girls are getting a bad deal.’ They thought I should be thrilled with that. I was thrilled we got a cheque, there was no question. I was thrilled with professional tennis. I am just so big on equality, if it was reversed, I would have thought that was wrong, too.”

Fifty years on and things have changed, for the better. The winners of this year’s singles events – men and women – will each receive £2.25m. However, achieving equity has been a long and slow process.

The US Open, under the threat of a boycott from King and the WTA, which she had helped form earlier in 1973, paid men and women winners equally from that year. But the Australian Open, which had dabbled with equal pay off and on, took until 2001 to achieve “official parity”. The French Open held out until 2006 and it was not until 2007 that Wimbledon finally agreed to pay the women the same as the men.

In the early days of open tennis King and the other leading women faced a battle to be accepted, as the 1968 prize money differential at Wimbledon showed. She says it was as if “they’re getting rid of us, the guys didn’t really want to share with us, they didn’t care if we had tournaments. They started dropping tournaments for the women. It was really a tough time from ’68 to ’70. Then we started the Virginia Slims Tour, that helped a lot. Then we started the WTA, our union, in 1973.”

It was then that Wimbledon really began to listen and act. “The first thing Jerry Diamond [who would later become executive director of the WTA] and I did was fly to London from San Francisco and sit down with the Wimbledon committee. We got them up from 30% to 70% [women’s prize money as a percentage of the men’s] in that one meeting. That was a huge improvement in one meeting when we went to Wimbledon, they were very kind, they announced it the next day. We were going in the right direction.”

Outside the grand slam events, the battle for equal pay is far from over. Of the top six “combined” events each year, only three – Indian Wells, Miami and Madrid – pay the men and women the same. A number of events played at the same time, such as Eastbourne, actually pay the women more, since their event is of a higher tier than the men’s tournament. However, total prize money across 64 tournaments on the ATP Tour is $137.5m, not including the four majors. The WTA’s total prize money, across 58 events, is $146m, but that includes the majors. Without them the total is around $80m.

King will be back at Wimbledon this year as one of the chairman’s guests, along with Laver, who won the men’s title in 1968 on his return from exile, having missed the previous five years after he turned professional in 1962.

In the 50 years since she won the third of her six Wimbledon singles titles, King has become a sporting icon. Her win over Bobby Riggs, immortalised in the Battle of the Sexes film released last year, catapulted women’s tennis into the spotlight. She has led the way in the fight for equality ever since, a task that continues, even now, with some male players piping up every now and again about getting “a bigger share of the pie”.

Perhaps those dissenters should spend a few minutes in the company of King. At 74 she has lost none of her charisma, her sheer will an inspiration to many, both in sports and in life, where she continues to be a leading advocate of gay rights. She has never been afraid to upset her hosts by discussing difficult subjects, as she proved in Melbourne this year when, during a speech recognising her award as “Australian Open woman of the year”, she said the tournament should remove the name of Margaret Court, Australia’s most successful ever player, from its third-biggest arena, because of her statements about the LGBT community.

When the topic of equal pay comes up in tennis, as it seems to do at Wimbledon every year, the main criticism from its opponents is that at the four majors, men play best-of-five-sets while women play best-of-three, an argument that ignores the time, money and sacrifices required to become a professional. King has always said she and the other women players would happily have played five sets if they had been asked but they never were. Now, in an era of ever-increasing physicality, when matches last longer and more top players are getting injured, she thinks the men should match the women.

“Now I think they should play two out of three,” she says. “It’s just too tough, my God, it’s too much. We’re wearing out our guys. I want them to play for forever, I don’t like it. It’s very different from when we were playing. It’s much harder physically than it used to be. I watch the guys, they get into five sets … it’s too long.”

King’s original dream was to have a joint organisation with the men, something that was shot down. “My plan A was always to have the men and women together, period, because it’s not just what we can do on the court, but also off the court to make the world a better place,” she says. “We also have more power together than divided. It’s simple. We’re so much more powerful together. If you are in business and you own a tournament where the men and women are together, and you want to sell your tournament, it’s worth so much more than having a single-gender tournament. It’s just better business.”

However, King does not believe it will ever happen. “I don’t see it,” she says. “The guys don’t want to share. They think we’re in the way: ‘Get out of here.’ They don’t think we have any clout. I think they’ll have to wake up again because they’ve had a golden era, especially with Federer and Nadal and Djokovic and Murray. Are you kidding — the four of them, amazing. That’s not going to happen every generation. We’ll still have one or two maybe, a lot of young guys coming up who are interesting, but they’re not Federer or Nadal.”

In 1968 King had an extra special reason to do well, with her parents, Bill and Betty, having made the trip to Wimbledon for the first time. Having beaten Britain’s Ann Jones in the semi-final, King was a big favourite against Tegart but caused her parents a few moments of concern when she jammed her wrist with a sliding bracelet and “went into shock” midway through the match. It did not matter, though. She held off Tegart to win 9-7, 7-5 (the tie‑break was not introduced at Wimbledon until 1971). She also won the women’s doubles that year.

King will hold court with Laver again over the Wimbledon fortnight, casting her eye over the current breed of players, those men and women who will fight it out for that first prize in a sport she has done more than anyone to bring into the 21st century. When the women’s champion collects her trophy, and cheque, she would do well to send a nod of thanks King’s way.