And by 21, she was done.
"I never planned or thought that was going to be it," Kournikova tells PEOPLE as she settles into her new role as a fitness trainer on The Biggest Loser. "My back really forced me to stop. It got so bad; I couldn't tie my shoes, literally. I would be in excruciating pain. I had been doing six to eight hours [of training] every day since I was 5 years old."
At first, doctors told Kournikova that recovery from major back surgery would take three months and then extended the prognosis to six months, and then a year.
"And then I just started going through a growing up period," Kournikova says. "It was very interesting to me to stop playing and to figure out, 'Who am I without tennis?' It was very difficult and scary as hell."
Away from the pro tour, Kournikova says, "I did everything from therapy to 10-hour walks on the beach to discover, and think, and try new things." She cites her family and longtime boyfriend Enrique Iglesias as part of her "support system, who have helped me figure out who I am."
During her time off, Kournikova says she learned to appreciate her pro career, even when others were criticizing her for not winning any singles titles. "I am not a robot," she says. "I have feelings, but I know that I did my best. I am very proud of what I have achieved."
Although she never made a formal announcement that she was retiring, Kournikova admits that her life was following a different path. In 2004, she started volunteering with the Boys & Girls Clubs of America and the USO.
"I loved getting involved in charity work and I felt like that's what my name is good for and that's what I wanted to channel it into," she says. "They showed me a reason to be and to be happy."
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Kournikova made the transition from volunteer and spokesperson to active trainer when she studied in Miami for her full-time fitness certification, which she ultimately received from the ISSA.
"I am so proud," Kournikova says of her new career. "The last time I felt like this, I was in Iraq and Afghanistan, Russia and Haiti on my charity trips, and then before that it was on the court! I am not doing this just to be on TV. At all. I am able to help and influence people's health and well-being. That is an amazing feeling."
The Biggest Loser airs Tuesdays on NBC at 8 p.m. ET/PT.