"I cried," Navratilova, 53, tells PEOPLE about the moment in February when a biopsy came back positive after a routine mammogram revealed a cluster in her left breast. "It knocked me on my ass, really. I feel so in control of my life and my body, and then this comes, and it's completely out of my hands."
She was diagnosed with a non-invasive form of breast cancer called ductal carcinoma in situ, or DCIS, which in her case was confined to the milk ducts and had not spread to the breast tissue. "It was the best-case scenario you could imagine for detecting breast cancer," Dr. Mindy Nagle, a good friend of Navratilova's (and her ob gyn), tells PEOPLE.
Navratilova had a lumpectomy and will begin six weeks of radiation therapy in May.
Dr. Shelley Hwang, a breast surgeon at UC San Francisco, says DCIS strikes almost 70,000 American women annually and accounts for about one-fifth of all new diagnosed breast cancers. "The prognosis of someone with DCIS is excellent," she says. "There's only a one-percent chance of anyone with this diagnosis would die of breast cancer."
The nine-time Wimbledon champion, who still plays tennis and ice hockey and competes in triathlons, says she is lucky, as she had not been getting regular checkups.
"I went four years between mammograms," she tells PEOPLE. "I let it slide. Everyone gets busy, but don't make excuses. I stay in shape and eat right, and it happened to me. Another year and I could have been in big trouble."
In her role as a health and fitness ambassador for AARP, Navratilova will be chatting live on the group's Web site on Thursday at noon.