Photo Credit Jonathan Exley

What is the No. 1 trait that has led to your success?

Warren Buffett's is focus—according to Alice Schroeder, author of the Buffett bio Snowball, who spoke, as I did, at a corporate event at the U.S. Open last week.

Getting ready to go on stage, I thought, what's my key trait? Curiosity, I guess. It keeps a journalist alive and open to ideas.

So I was innately curious to interview Billie Jean King that afternoon at the Open. As soon as we took the stage, to the blare of Elton John's "Philadelphia Freedom" and a standing ovation for the tennis champ, I asked her what is her No. 1 trait. "I love people," she replied.

It wasn't the answer I expected, and I didn't understand it at the time. Though it is an appropriate one for this audience, which was convened by Adecco (AHEXY), the global staffing giant. ("Did you customize your answer?" I asked. King swore she did not.)

For the next hour, King rushed the net, interview-wise—volleying answers every which way as I tried, to no avail, to control the game. "I want to talk tennis," I said at one point, "and you want to talk business." It's like that with King: You never know what you're going to get.

"Relationships are everything, " she said when I asked who has impacted her career. There was the moment when she was 11 years old, in Long Beach, California, when Susan Williams, her childhood pal, suggested they play tennis—to which Billie Jean replied, "What's tennis?" In no time she decided she would be the world's No. 1 player. It was her parents—her firefighter dad and her mom who sold Avon (AVP) to help pay for her to enter tournaments—who showed her that success is about more than winning every match. More than caring what the score was, they always asked, "Did you do your best?...What did you learn?"

When she reached the top, King's higher calling came off the court. She led the fight for equal prize money—threatening in 1972, when she won the U.S. Open, to bow out the next year if the women's prize money didn't match the men's. In 1973, the U.S. Open became the first major tournament to award equal prize money. She beat Bobby Riggs in the Battle of the Sexes—a star turn, in 1973, for women's equality. But the bigger deal was that she led the creation of women's professional tennis by rallying a bunch of other renegades, lining up Philip Morris (MO) as a sponsor (remember the Virginia Slims tour?), and organizing the WTA players union.

She led "because people asked me to," King says, noting, "Followers choose leaders." She was well-paid for her leadership—the first woman athlete to earn six figures. But she also knew what to do with the money: In 1974, after Bob Hope and Tony Randall handed her a $5,000 check to give to her favorite charity, she figured, Why not start our own? So she did: the Women's Sports Foundation, to help women and girls through sports.

Today, at 67, King and her partner, Ilana Kloss, own World Team Tennis, a co-ed tour that is all about gender equality. Even though she's shy, she's built friendships with an eclectic array of powerful people, from Elton John (helping him raise money for AIDS research) to Warren Buffett (BRKBA). She cold-called Buffett five years ago, told him she wasn't interested in his money, and flew to Omaha to meet him.

And now the tennis establishment she once battled calls the venue of the U.S. Open the Billie Jean King National Tennis Center.

On Saturday, I was back there and swung by King's suite. What a kick to see Rosie Casals there—she and King were the U.S. Open doubles champs 34 years ago. Into the suite walked a tall fellow with a strong build, a baby face and an obvious respect for our hostess. It was Brett Connors, Jimmy's son, now 32. As I watched King work the room, I saw that "loving people" has indeed equipped her to expand her power. "Physical athletic ability is a dime a dozen," she said to me, adding, "I'm happier in my career now than ever."

Click here to view more from the Fortune Most Powerful Women-Yahoo (YHOO) Shine "Power Your Future" series.

For updates on the Fortune Most Powerful Women community, like us on Facebook and follow us on Twitter.