"So Margaret played him Mother's Day in 1973 and lost (6-1 6-2). it's called the Mother's Day massacre and I just thought, 'Oh no!' "
King needed no second bidding, and the famous "Battle of the Sexes" match came to fruition on September 20 in the Houston Astrodome.
"I thought it would set us back 50 years if I didn't win that match," said King. "It would ruin the women's tour and affect the self esteem of all women."
The entrances -- King on a gold litter in the style of Cleopatra, Riggs on a rickshaw pulled by women models in skimpy outfits -- added to the theater.
Once the match started, King, at 29 and the peak of her powers, made her opponent eat his earlier words and boasts.
With a winner-take-all $100,000 check riding on the outcome, Riggs lost his nerve and was beaten 6-4 6-3 6-3.
"The drop shot and volley heard around the world," said Britain's Times newspaper as an estimated global TV audience of 50 million watched the rout.
King and Riggs embraced at the end, and became friends off the court until his death in 1995 of prostate cancer.
Prior to the WTA's formation, King had realized the significance of legislation passed through the U.S. Senate and House of Representatives and signed into law by President Richard Nixon in June 1972.
Title IX made it a requirement under law for male and female students to be afforded equal federal funding in their high school and college studies.
"Before that, young women were not getting anything, there was gender quotas -- like 5% in the medical school at Harvard -- there were really terrible gender quotas and also women could not get an athletic scholarship in the States," said King.
"Now because of Title IX, women from all over the world can go to our American colleges on a scholarship and get grants. It's very powerful because it's about equality in education and activities, and sports comes under activities."
According to 18-time grand slam singles winner Navratilova, King took advantage of the mood of the moment to push through changes which were ahead of their time.
"Billie Jean, she just pushed the clock forward, she sped up the process," Navratilova said.
"Any progress is measured by jumps, and that was one of those jumps that pushed the clock forward and allowed us to move forward as women athletes and to make a career out of it so it wasn't just a hobby. "
King was to play competitive singles for 10 more years after her 1973 heroics on and off the court, but injuries took their toll.
Her final grand slam singles triumph came at Wimbledon in 1975, her sixth success on the grass at SW19, but victory in the women's doubles at the 1980 U.S. Open competed her set of 39 major titles overall.
A firm believer in the team ethic, she played for and captained the U.S. in the Wightman and Federation Cup competitions, while King and her husband Larry were founding partners of World Team Tennis in 1974.
The format of that competition sees men and women playing a five-set competition in a mix of singles, doubles and mixed doubles.
It has proved a successful formula, with the franchises playing to big crowds across the United States.
For King, the whole ethos of the event sums up her attitude to life and equality.
"That's the way I want the world to look: men and women working together, championing each other, helping each other, promoting each other -- we're all in this world together," she said.