Billie Jean King can still remember exactly how she felt as she walked to the net.
"He said to me, 'I underestimated you,' and all I could hear in my ear was my father telling me ever since I was a small child, 'Always respect, never underestimate.' Riggs should have listened to my father."
And she chuckles, the memory of victory on a tennis court in 1973 as fresh now as it was then.
Billie Jean was certain she was going to win, even if she can't remember it now
It was a particularly sweet moment for BJK. Her opponent Bobby Riggs had been trumpeting that there was no room for women on the tennis court, that they could never match the men, just as King and her peers were battling for social change, equality, in general, and for the women's game to be taken seriously, in particular.
Perhaps the odds weren't evenly stacked. After all, BJK was one of the top players in the world, with 10 single Grand Slam titles in her handbag. Riggs was 55, well past his prime. But even so, did she know she was going to beat him?
Bobby Riggs was trumpeting very loudly about his superiority - before the match, anyway
"I can't say I did, although Rosie Casales who was commentating, reminds me she came into the locker room 20 minutes before the match and asked me, and I just nodded,” says Billie, laughing that she can’t remember any of this.
"And my brother told me only the other day, apparently I called him up the night before and said, 'You can bet the house, I'm going to win.'
"He was a Major League baseball player, so he went and put a betting card on his locker, and took wagers from everyone. That night, he went to the hotel, got these chicken wings and ate himself silly, and just thought, 'I'm going to make so much money tomorrow', plus he got to tease the guys, which was obviously more important."
As well as a glorious battle watched by an estimated 90 million people worldwide, BJK remains highly aware of the significance of her victory.
“I knew what it meant, that's why it was tough, I knew it was about social change and justice. I knew it was going to make the country crazy, sometimes you just intuitively know things. It was a mind-boggling time.
“As a leader, I think it's really important to judge the emotions of the time, what people are thinking and feeling, and it was the height of the women's movement, it was pretty clear what was going on."
Billie Jean King had the fire in her belly... "tennis was my platform, but I was always thinking about other things"
That’s quite a load for young shoulders? She laughs. “Well, you can make it as big or as small as you like. I just knew it would ignite people’s feelings, and it did.
“Even now, every single day I walk outside the door, people come up and tell me where they were, or they just talked to a grandparent, they all have a story to tell. One man told me he threw his TV out of the window he got so mad…”
Billie was always as much of a firebrand off the court as she was ruthless on it, causing ripples with her championing of the women’s game, as one of the founding members of the WTA, 40 years old this year.
"I loved tennis, I had a passion to play, but it was also my platform. When I was in the dressing room, I was always thinking about other things. Arthur Ashe and I were very much the same. I think we were products of our time.
“I think if there's a need or a want, someone appears almost."
33 years after her last of her 39 Grand Slam titles, Billie Jean King is evidently as much of a thinker as ever, rattling off the differences in pay between men and women, and the role her fellow sports-people have in changing the world.
“Sport's a microcosm of society,” she muses. “The difference is we don't get to do it for very long. If you're a singer, a writer, you can go on forever, but sportspeople, we have to grab our chances.”
I can’t leave her without asking what she thinks of Andy Murray’s chances this year at the All England club? She’s diplomatic… “This is his best chance. And he's evolving…"
“Yes,” I muse, “He's showing his soft side these days We're taking him to our hearts now. In fact, you could say he's won already?"
She looks almost disbelieving. "You could…" she says slowly, and that gleam that should have warned off Bobby Riggs comes back into her eye. “But you're talking to an athlete, baby."
The Battle of the Sexes is in UK cinemas from today. Watch the trailer here…