AMNA NAWAZ: Today, some women's tennis players are among the highest paid athletes in the world.

But that wasn't always the case.

Equal pay was a years long battle, hard fought by the same women playing the game, including the legendary Billie Jean King.

I met up with King at the U.S. Open recently, as the tournament marked 50 years since it became the first sporting event in U.S. history to offer equal prize money for men and women.

It's been decades since she played on these courts and literally changed the game for women.

BILLIE JEAN KING, Former U.S. Tennis Champion: Well, we're microcosm of society anyway, sports are, so -- but at least we're going in the right direction.

AMNA NAWAZ: But Billie Jean King is still everywhere at this year's U.S. Open, as the tournament marks 50 years of equal pay.

And she's not nearly done pushing for change.

BILLIE JEAN KING: Women's tennis has always been the leader in women's sports, so we have a responsibility and an opportunity to help make this world a better place.

AMNA NAWAZ: It was a different tennis world in which King made her name, winning 39 Grand Slam titles between 1961 and 1979, with a number one ranking for six years.

It was at the U.S. Open in 1972 when King clinched the women's singles title, then made a revolutionary demand.

BILLIE JEAN KING: I just remember sitting in the media conference after I'd won.

I don't know what came over me, but inside my heart and soul said, I don't think we will be back next year unless we have equal prize money.

And I said it very quietly.

I'm not ranting and raving.

And the media goes, what?

And I said, I haven't talked to the women yet about it, but I'm pretty sure that they will go along with it.

And, of course, inside, I'm going, what have I done?

Because what if the women don't get behind?

I'm like, I thought they would.

AMNA NAWAZ: King made sure they did, rallying more than 60 other women players in London's Gloucester Hotel in the summer of 1973 to get them to band together and form the Women's Tennis Association.

And you told someone to literally stand at the door BILLIE JEAN KING: Yes.

Don't let anyone leave.

I said, don't let anybody out.

Lock the doors.

(LAUGHTER) BILLIE JEAN KING: We have to have an association.

AMNA NAWAZ: It was king the so-called Original 9 who laid the foundation three years earlier, at a time when some men earned as much as eight times as the women.

This groundbreaking group signed $1 contracts each to take part in the first women-only tournament.

And in June, King and the other WTA founders returned to the famed London hotel where the WTA first formed 50 years ago, forcing equal pay at the U.S. Open.

It feels both like a long time ago and also not that long ago at all.

How does it seem to you?

BILLIE JEAN KING: It seems, in some ways, recent, because I can remember everything clearly.

In 1972, I got $10,000 for winning.

Our tour was only in its second year then, so women's professional tennis had just gotten started.

AMNA NAWAZ: King too was just getting started, off the court, a private war, convincing one corporation to help close the prize money gap.

ANNOUNCER: The Tennis Battle of the Sexes.

AMNA NAWAZ: On the court, a very public, so-called Battle of the Sexes, taking on and beating Bobby Riggs in 1973.

An estimated 90 million people tuned in, tennis' most watched match to this day.

King went on to rack up 12 singles Grand Slam titles.

The three other Grand Slams followed the U.S. Open's lead on equal pay.

But that took decades.

Even today, across women's sport, the pay equity fight rages on.

But the fact that it's taken 50 years to get to a place where this is not the norm?

BILLIE JEAN KING: When you have talked to CEO or companies, do you invest in women's sports as much as you do in men's sports, and they usually get very quiet, but a lot of times they will say: I hadn't thought about it.

So just putting the thought in their heads is a start.

AMNA NAWAZ: You have always -- since you were 12, you have found a reason to speak out for what you think is right.

Do you see that same kind of leadership and willingness to speak out among this new generation, the younger players today?

BILLIE JEAN KING: Oh, absolutely.

Venus Williams fought for equal prize money through the years.

One thing that we stressed in the WTA and the older players at that time when we started was, this is a platform.

We have an opportunity.

We actually have an obligation to help make this world a better place.

AMNA NAWAZ: Mary Joe Fernandez, a former world number four, says King's contributions to the sport weren't without a cost.

MARY JOE FERNANDEZ, Former U.S. Tennis Champion: She had to sacrifice so many tournaments and her practices and her tennis career for something bigger than herself, which is why I think she's beloved.

But now I think there's so much more money, everybody's so much busier.

Tennis is not just tennis.

It's the physical part, it's the mental part, it's the nutritional part.

A lot revolves around all these athletes, and I don't feel like the top, top feel like they have the time to really dedicate themselves like Billie Jean did.

AMNA NAWAZ: What do you think about that?

MARY JOE FERNANDEZ: I understand it.

I totally understand it.

But I'm hoping, down deep, that if Venus ever retired, she would sort of take a little bit of the mantle and takes that responsibility, because she really helped women's tennis.

AMNA NAWAZ: I asked 29-year-old American Jessica Pegula, now number three in the world, about how her generation views that responsibility.

I wanted to get your thoughts on whether you individually feel pressure or responsibility as a leader in this generation of players to continue that advocacy work.

JESSICA PEGULA, Professional Tennis Player: I think, when you're younger, you're just playing and you don't really care, and you're just, oh, this is fun, this is my dream, and all this stuff.

And then you start realizing how important it is to make an impact on your sport and leave with some sort of legacy.

It's a very exciting time in women's tennis.

I think that we are improving and we're moving forward, and, hopefully, we keep seeing that, yes, and keep pushing for what we believe in.

AMNA NAWAZ: Women's tennis, like other sports, is also grappling with how and when transgender athletes can compete.

King herself lost endorsements when she was outed as gay in 1981 and has been a champion of LGBTQ+ rights.

Her views on this issue, she says, are still taking form.

BILLIE JEAN KING: I do think, in certain sports, we're going to have to take a hard look, because, especially timed sports, track and field swimming, each sport, I think, is making up their own rules, from what I understand.

I'm trying to -- I keep learning.

I keep talking to scientists.

Please help me to understand what's the right thing to do.

One thing for sure is, when they're young, let everybody have fun and play.

Once you get to probably high school and you start having elite sports, where it matters, like going to the Olympic, to be a professional athlete, then I think you really have to have rules.

I'm still trying to learn more and more all the time.

I'm not that emphatic.

I'm trying to figure it out.

AMNA NAWAZ: But you think that should change after high school?

BILLIE JEAN KING: If a male gets through puberty, they have a bigger skeletal system, their heart's bigger, everything.

So, these are the things you have to take into consideration.

MICHELLE OBAMA, Former First Lady: Ladies and gentlemen, my friend the incomparable Billie Jean King.

(CHEERING AND APPLAUSE) AMNA NAWAZ: As her views and the sport continue to evolve, she's enjoying the spotlight put on the 50th during this tournament and turning her attention to the future.

I asked King, who turns 80 this year, about the many ways conditions for women have improved since she stopped playing.

Does any part of you wish that you were playing today?

BILLIE JEAN KING: Yes, I would love to be able to play today.

That would be fun, because they're better than we were, and I would always want to get better.

They're living the dream.

That's exactly what we wanted.

AMNA NAWAZ: Her fight on the court locked in her place in tennis history.

Her work off tethered King's story to our nation's.

In 2009, she was awarded the Presidential Medal of Freedom by President Barack Obama.

There's now an effort under way to award King the Congressional Gold Medal.

Only 11 athletes have ever received the award, but King would be the first woman.

GEOFF BENNETT: I love that she says she still wants to play.

AMNA NAWAZ: Oh, yes.

GEOFF BENNETT: That's fantastic.

AMNA NAWAZ: Such a trailblazer, still going strong.

GEOFF BENNETT: Great interview.

AMNA NAWAZ: Thank you.