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Crouching at the start of the women's 1,500-meter in Sydney on Sept. 30, 2000, Suzy Favor Hamilton scanned the runners lined up beside her. Earlier that year, the three-time Olympian had posted the world's fastest time in her event; her lean form had graced magazine covers from Fitness to Outside and she had a worldwide fan base, cheering loudly from the stands. The Olympic gold was hers to lose.

Yet something was wrong. "I was thinking about a friend dying of cancer," Suzy recalls. "I should have been thinking about winning." After a strong start, she was leading the pack going into the last 200 meters when first one, then another, then a third runner passed her. Suddenly she collapsed, her body doubled up as medics raced onto the track. "I thought to myself, 'Get up,'?" she recalls. "'They don't want to see a loser.'"

Commentators speculated about dehydration, but what had felled Suzy was something else entirely. "I couldn't face the world anymore," she tells PEOPLE. "I closed my eyes and pretended to pass out."

For years, Olympic-watchers and running fans have wondered about the mysterious mid-race breakdown that knocked a sport's "It girl" from her perch. Now, for the first time in a national publication, Suzy, 40, tells PEOPLE her failure at Sydney was the result of a long-buried depression that, at her lowest point, drove her to thoughts of suicide. "Depression," says Suzy, now a married mom of a 3-year-old in Madison, Wis., "is something that needs to be talked about."

The benefits of exercise in both guarding against and helping alleviate symptoms of mild to moderate depression are well known. Elite athletes, however, sometimes believe they are immune to depression or, when faced with it, use their sport as a crutch, experts say. "Exercise can be very positive," says Ross Flowers, a Chula Vista, Calif.-based psychologist who works with Olympic runners. "But it can also hide underlying issues which may require psychotherapy or medication."

Suzy was well acquainted with emotional suffering. Growing up in Stevens Point, Wis., she saw her older brother Dan repeatedly hospitalized for bipolar disorder. "It was never talked about," says Suzy, who used running to help her forget her troubles. She started competing at age 10 and soon was logging six miles a day. "If I didn't run, I'd be moody," she says. "It became my drug."

During and after college at the University of Wisconsin, she established herself as one of the nation's premier middle-distance runners—setting U.S. records in the 1,000- and 800-meter events and becoming one of only two American women to have run the 1,500-meter in under four minutes. She maintained her laser-like focus and sunny demeanor even after a family tragedy in 1999, when Dan killed himself at age 37. "She was the fiercest competitor," recalls former pro runner and training partner Michelle Ave. And while Suzy had stumbled in several races before 2000, "I never was aware of any depression," says former coach Peter Tegen. "She was a bubbly, friendly and outgoing young lady."

Even after the Sydney disaster, Suzy kept running—professionally, on the track and away from her problems. Married to college sweetheart Mark Hamilton and living in Madison, she busied herself helping Mark build a real estate business and getting ready to start a family. But after the birth of daughter Kylie in 2005, she was hit with crippling postpartum depression. Battling obsessive thoughts about driving her car off the road, she tried to exercise—and couldn't. "I was on the floor in front of my treadmill, curled up, crying," she says. "And then it hit me—'Suzy, you have a problem.'"

After a year of therapy and medication—she still takes an antidepressant—Suzy finally achieved an equilibrium that had eluded her all her life. "Now," she says, "when problems arise, I take a moment to react rather than running from them." Her main sports these days: amateur speed-skating and charity runs. As for that empty place on her mantel, she says, "It would be fabulous to have that gold medal, but if I didn't live this journey, I wouldn't be where I am today."

www.nimh.nih.gov/health/topics/depression