Twenty-five years ago, Andrea Jaeger was the terror of the women's tennis tour. While rising to No. 2 in the world, Jaeger admits she sometimes screamed at linesmen and was cold and standoffish to competitors in the locker room. Some called her Superbrat.

Cut to the present: The former tennis star, surrounded by a gaggle of young cancer survivors, strolls the halls of Cincinnati Children's Hospital. At each room, the gang stops and, laughing and swaying, belts a splendidly off-key version of "Somewhere over the Rainbow" or another sing-along. In their beds, other kids, still patients, giggle. "She's truly amazing," says Timothy Cripe, an oncologist at Cincinnati. "You can't help but smile when she's around—even if you do have cancer."

You can't help but notice a couple of other things about Jaeger too, and not only that her world view has changed dramatically: On Sept. 16 the former tennis terror became, officially, Sister Andrea, an Anglican Dominican nun. Now 41, she has dedicated her life to the care of kids in need, visiting dozens of hospitals each year and running a camp for kids with cancer in Colorado.

Sister Andrea says that even when she was on the tour, there was a kinder, gentler, more spiritual soul lurking beneath. Although never outwardly religious, and despite the fact that her parents "didn't own a Bible," she says she always felt that "God had a plan" for her.

Her father, Roland, a Lincolnshire, Ill., tavern owner, also had a plan, which at the time took priority: A tennis natural since the day she picked up a racquet at age 8, Jaeger won a slew of junior tournaments and, pushed by her father, turned pro at 14. Fast on her feet, with a devastating arsenal of shots, the 5'3", 105-lb. Jaeger "was a tremendous little powerhouse," says tennis commentator Bud Collins. "She was just feisty, a real fighter." Within two years she was ranked No. 2—and widely disliked by fellow competitors, who found her cold and unfriendly, an attitude not totally discouraged by her father. At about that time, she says, she had a revelation that she felt was too private to tell anyone. As a tennis pro, she visited a hospital to cheer up kids, and "I knew this is what I was supposed to do when I grew up," she says. "I was going to help kids who were stuck in the hospital."

Jaeger's tennis career ended at the French Open in 1985, when she blew out her right shoulder. Seven surgeries later, she retired in 1987 at the age of 19. Although she had won $1.4 million, Jaeger took a job as a receptionist to keep busy. She moved to Aspen in 1989 to start a foundation for kids with cancer and invested her life savings. "For me, starting the foundation and putting all my money in was such a natural course because it was my peer group in a sense—every child wants to be connected," says Jaeger. Today her Little Star Foundation (www.littlestar.org), which she runs with best pal Heidi Bookout, 51, provides programs for more than 8,000 children yearly who are seriously ill, abused or at risk. The only tennis she plays these days is with the hundreds of kids who visit her cancer camps.

Although Jaeger obtained an online associate's degree in theology in 2001, she didn't hear the call to become a nun until earlier this year. It came in a dream. "I dreamt I was getting a tour of a monastery. My tour director was St. Catherine of Siena, whose mission was to help the sick, the poor and the suffering." She was reluctant at first, but when she discovered she could continue her online studies and still run the foundation, she accepted the calling. "My biggest concern was the foundation," says Jaeger, who labors to raise the funds for programs that cost $4.3 million yearly.

Besides taking on new studies, Jaeger is moving forward with the 200-acre camp her foundation is building outside of Durango. Her Order doesn't live in convents; home for her is an adobe house nearby. Although as an Anglican she's free to marry, the never-wed Jaeger has taken a vow of purity. "When people ask if I miss tennis," says Jaeger, "my answer has always been, 'No regrets. God wanted me to do something else, and it happened to be helping children with cancer. I love what I do.'"