With 39 Grand Slam titles, Billie Jean King is a former world No. 1 tennis player and a pioneer for equal opportunities and pay for women in tennis and beyond, as well as for LGBTQ rights. King won the highly publicized "Battle of the Sexes" in 1973. against former men's champion, Bobby Riggs, in a landmark moment for women's sports and society. She continues her work for women's equality and leadership through her eponymous foundation and is active in World TeamTennis.

People today are drawing on a tradition of sports activism in which you played a big part. When you were starting your push for pay equity, did you expect to be where we are now? Did you expect to be further?

I expected to be at least where we are. I’m big on equal pay for equal work. Money matters. Money talks. Money gives you opportunity. A quarter of single parents are men, and three-quarters are women. And when women make less money, they take less money home for their family. And it is baloney. It has to change.

When did you first start speaking out?

Well, it’s a slow process. First we fought for $14 a day in amateur tennis. Then I fought for open tennis — pro tennis [for] men and women. Finally we got it in ’68. So we got paid. But at the first [open] Wimbledon tournament, Rod Laver and I both won in our events. He got 2,000 pounds, and I got 750 pounds. When I got my check, I looked down and went, “Oh, criminy. Now we’re going to have to fight for something else.”

So you didn’t realize you were [going to get] different purses?

Absolutely not. So you just keep working on it. Do you know the history of women’s tennis? We had nine of us in 1970 that signed a $1 contract with Gladys Heldman, and that’s the birth of women’s professional tennis. We started the Virginia Slims tour. We had no infrastructure. We had no money. We were giving up everything. We could have never played at Wimbledon again. Never played a tournament again. We didn’t care. Our goal was that any girl born in the world, if she’s good enough, will have a place to play, would be appreciated for her accomplishments — not just her looks — and be able to make a living, because we had known what it was not to make a living. I got all of us organized, and we spread out, talking it up, trying to influence others. Finally we formed the Women’s Tennis Association. So we had our union.

In 1973, former men’s champion Bobby Riggs followed me around and asked me for this match. And I said no. But then Margaret Court — it was a lot of money — said she was going to do it. I told her, “Margaret, you have to win. It’s not just a tennis match. It goes way beyond.” But she lost. This is just after Title IX. We just started the Women’s Tennis Association; we’re playing this Virginia Slims tour. We’re only in our infancy. But anyway, the point is, she lost. Now I have to play him. I don’t have a choice. I knew it was pivotal. It would help tennis, women’s sports.

That’s a lot of pressure.

Oh, it was a huge amount. Ninety million people saw it. And it wasn’t just the United States. It was the world. The media were all excited because I’m playing against a guy. If you actually listen to what [sportscaster] Howard Cosell said when I was being brought out, he talked only about my looks. But when Bobby Riggs came out, he only talked about his achievements.

And what did it feel like when you won?

Such relief. Oh, God. [Riggs] jumps over the net, and he whispers in my ear. He goes, "I underestimated you." It's really amazing what came out of that match. Women gained a lot more self-confidence. I just had a woman today come up to me and tell me, "That changed my life." And that happens a lot. The women are very exuberant, usually. Fired up. The men are very subdued, usually reflective. Sometimes they have tears in their eyes. They say it changed their life because, for the first time, they actually thought about girls and thought about their lives. And they have a daughter now. They say it changed how they raised them because of that match. Actually, President Obama, when I met him, he said he watched it when he was 12. And it helped him raise his daughters. It changed him; it made him really think.

I have this saying: Pressure is a privilege. Usually if you have tremendous pressure, it’s because an opportunity comes along. I remember thinking about this, actually, when I was at Centre Court at Wimbledon. And I said, “All right. You’ve been dreaming about this moment. Is it a lot of pressure? Yeah. But guess what? It’s a privilege to be standing here.” Most of the time, in work or play or anything, if you really think about it, usually it’s a privilege. That I-want-the-ball feeling. Not “please double-fault.” Give me the ball. Give me the problem to solve. Let’s figure this out. Let’s go.

This interview has been edited and condensed. It is adapted from KK Ottesen’s latest book, “Activist: Portraits of Courage,” which will be published in October by Chronicle Books.