SARASOTA, Fla. -- Martina Navratilova has won 18 Grand Slam singles titles and 31 Grand Slam doubles titles, but she is now in for one of the most important fights of her life.

The Czechoslovakian-born tennis legend has announced that she has breast cancer.

Navratilova, who lives in Sarasota, Fla., has been an American citizen since 1981 but renewed her Czech citizenship in 2008. She is 53. Shortly after being granted her U.S. citizenship, she came out publicly as a lesbian, one of the first professional female athletes to do so.

Now a health and fitness ambassador for AARP, Navratilova is a pescatarian and still active in tennis, plays ice hockey and competes in triathlon events. She admits her healthy lifestyle and status created a certain complacency when it came to following through with her annual checkups.

"I went four years between mammograms," she tells PEOPLE magazine. "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."

Luckily, the cancer was detected at its earliest stage. She has already undergone a lumpectomy and will begin six weeks of radiation therapy in May. Her doctors say her prognosis is excellent, but Navratilova is taking her situation very seriously and wants other women to also take notice.

She described hearing the news after a routine mammogram this past February as, "my own personal 9/11." She announced her condition on Wednesday.

"It knocked me on my ass, really," Navratilova told the magazine. "I feel so in control of my life and my body, and then this comes and it's completely out of my hands."

Her test results uncovered ductal carcinoma in situ, or DCIS, a type of non-invasive cancer that is confined to the milk ducts. It has not spread to the surrounding tissue.

Her case proves that cancer can happen to anyone.

She issued a statement through the AARP, which read in part, "This is a huge wake-up call for me and just goes to show no matter how much you watch what you eat or exercise you just never know.

"Here I am, the health and fitness ambassador for AARP, speaking to millions each month about staying healthy and I let my annual check-ups fall to the bottom of my to-do list. It's not all about eating right and exercising. Preventative steps can make just as much, or in some cases more, of a difference. Getting my mammogram literally saved my life."

She encourages everyone to get their annual mammograms.