No one can accuse Andrea Jaeger of lacking enthusiasm. Ask about the game she entered in 1979 as a 14-year-old professional athlete and she'll talk about the amazing opportunities the tennis circuit gave her. Ask about her autobiography, First Service: Following God's Calling and Finding Life's Purpose, or her life as a Christian, and she'll beam.

But ask what it means to help kids with cancer and there's no stopping the passion from spilling out of her. Andrea is cofounder and executive director of The Silver Lining Foundation, which uses a ranch in Aspen, Colorado, to help children with cancer and other life-threatening conditions.

"Being of service to God is as much about letting go as holding on," she says.

For Andrea, that first step started with a tennis racket. Andrea grew up in Illinois as the daughter of German immigrants. Her father taught her how to swing a racket and hit a ball on the family home's driveway. Soon she entered local tournaments and quickly climbed her way to top status in the national under-12 division. By 13 she was clobbering opponents two and three years older.

As Andrea won more and more matches, sportswriters began to call her the "child phenomenon." The opportunity to turn pro came when she was 14—an unheard-of age for tennis pros in the early 1980s—and she took it.

Shocking opponents, sports commentators and sometimes even herself, Andrea became the number two female player in the world. She traveled across the globe to play all the major tournaments such as the Australia Open, the French Open and the Virginia Slims Classic. She even upset Billie Jean King at Wimbledon in 1983.

But Andrea discovered that tennis, as much as she loved the game, would never satisfy her deeper desire for connection and service. Though her family was not religious, Andrea always had a sense of God's presence in her life from as early as age 6. She talked with Him regularly, and believed she heard Him answer. Spiritual visions sometimes led her to show compassion to people on the tennis circuit who were hurting. Still, she rarely talked about her faith.

When Andrea was 16 and getting ready to play in New York City, an unusual idea popped into her mind: she wanted to buy a few toys for some of the children at Helen Hayes Hospital. She went to a store, loaded up with dolls, games, and stuffed animals and expected to walk into the hospital ward as Santa Claus. Instead, Andrea found that the kids gave her a gift she could not have imagined: her calling.

"A boy without hands challenged me to a video game and before we knew it we were having so much fun laughing that soon the room was filled with other kids, families and nurses," Andrea says of that first visit, the first of many. "I knew at that point that I wanted to help kids."

And she did. When she turned 19, she suffered a shoulder injury that virtually ended her professional tennis career and made her evaluate her life, her faith, and the joy she found helping children. While out of the spotlight, she realized that some people had put a value on her only when she was a healthy player winning matches. That disturbed her. So she began to research children's charities and decided to establish a foundation that would make a difference in the lives of the children who had inspired her.

"During my years on the professional tennis tour, visiting children in hospitals was one of my favorite things to do at tournaments," she says. "Children are models of hope and courage. Watching them fight for their lives on a daily basis inspired the premise on which the foundation is based: Kids will have an opportunity to regain some of their missed childhood—a childhood stripped by the harsh realities of cancer."

Now Andrea spends her time organizing horseback riding trips, gondola rides up Aspen Mountain, and hikes in the Rocky Mountains. Hundreds of special-needs children have enjoyed camping experiences at Silver Lining's Benedict-Forstmann Ranch. When Andrea is not spending time playing with the kids at the ranch, she's speaking at fundraising dinners, youth events, schools, churches, and, of course, children's hospitals throughout the world. (Last year, she sent 575 copies of her book as gifts to sportswriters and media representatives at Wimbledon, as well as 125 to children with cancer she had taken to the tennis tournament. She is donating all proceeds from the book to the Silver Lining Foundation.)

"Wendy Turnbull, an Australian (tennis) player and Christian mentor, once told me when you love something and are called to it, it's not a sacrifice," Andrea says. "We don't know if some of these children will make it 20 days or 20 years. I just know it's a life calling for me to be with them, an extension of the wonderful things God has done for me."