updated 09/27/2011 AT 12:00 PM ET
•originally published 09/27/2011 AT 2:45 PM ET
By the time Moscow-born Anna Kournikova was 8 years old, the tennis prodigy had already signed with powerhouse sports agency IMG. By 9, she moved to the U.S.A. to train with legendary coach Nick Bollettieri. By 18, she ranked No. 8 in the world in singles.
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.”