By Blair Henley

(June 13, 2012) -- It’s been nearly 30 years since Billie Jean King last played a professional singles match, but the 68-year-old’s impact on the game of tennis has only been magnified since her retirement.

Thanks to her extensive list of off-court accomplishments, which includes founding the Women’s Tennis Association and the Women’s Sports Foundation, King has become a respected advocate for women in sport.

And with this year marking the 40th anniversary of the Title IX legislation that prohibits gender discrimination within "any education program or activity receiving Federal financial assistance," King’s achievements were highlighted in yesterday’s interview with Andrea Mitchell on MSNBC. There the tennis star credited her historic Battle of the Sexes win over Bobby Riggs with helping the general public embrace a shift toward women’s equality.

(Watch: BJK on MSNBC's Andrea Mitchell Reports)

"What I really wanted to do by beating Bobby Riggs was starting to match the hearts and minds of people to the legislation of Title IX,” she told Mitchell. "I hope that it was a good kickoff because you know, in 1973, we couldn't even get a credit card on our own without a guy signing for us or with us."

In an age where the women’s U.S. Open champion takes home a whopping $1.8 million, it’s hard to believe that King had to capture 30 tournament titles in order to become the first woman to make $100,000 as a professional tennis player in 1971.

Since then, King has been a driving force behind the move to award equal prize money to both genders at all four Grand Slam tournaments. In fact, after winning the U.S. Open 1972 (and receiving $15,000 less than the men’s winner) King announced that she would not return the next year if the female winner didn’t receive equal pay. At the mercy of one of its biggest stars, the USTA complied.

But think again before you assume the move is driven by the dough.

"People always say it's about the money," King said. "It's not about the money. It's about the message. That's what Title IX is about. Now we have 54 percent of women enrolled in colleges and universities and our job is to continue to provide opportunities. We have 1.3 million less opportunities, girls do, in high school to be in sports than boys."

Despite eight knee surgeries and her approaching 70th birthday, King is clearly showing no signs of slowing down.

"We still have a long way to go," she said. "But the important thing is that we keep the girls in the game, particularly with obesity."