Martina Navratilova

Martina Navratilova has breast cancer, but she expects to make a full recovery.

"It was my personal 9/11," the tennis legend told Good Morning America's Robin Roberts (herself a breast cancer survivor) of the February day she received the diagnosis. "I was shocked because I was so sure [the calcification] was benign, and I found out and I was devastated. Physically, I couldn't think, I couldn't move. I was useless."

Navratilova, 53, was diagnosed with ductal carcinoma in situ (DCIS), a non-invasive form of breast cancer. Because the cancer is in one breast and has not yet spread, there is a "very small chance" of a relapse.

Although she was surprised by the news — because "I've been healthy all my life" — the 18-time Grand Slam champion said she only has herself to blame for taking four years between mammogram appointments. She said she was feeling "100 percent fine" at the time of the diagnosis.

Watch clips of Martina Navratilova

"I went every year and then I just let it slide. I moved and I changed doctors and I was like, 'Oh, I'll get to it.' I did not realize it had been four years," she said. "The reason I want to speak about this is to encourage women to have that yearly check-up because I let mine slide. I'm lucky that this DCIS had apparently just started within the last year. If it had started three years ago, I would be in deep trouble now. There would be chemotherapy. The cancer would've spread."

As a health and fitness ambassador for AARP, the former world No. 1 will participate in a chat Thursday to discuss her diagnosis and encourage screenings.

Navratilova hasn't let cancer slow her down: She participated in the Hit for Haiti charity tennis match and played hockey in the three days leading up to her a lumpectomy last month, and did the biking portion of a triathlon two weeks after the procedure.

In May, she will have six weeks of radiation, which she hopes to undergo in Paris so she can continue her Tennis Channel commentating duties for the French Open.

After the radiation, there should be a less than 10 percent chance of the cancer returning, she said.

"I'm OK and I'll make a full recovery," she said.

Watch Martina's announcement: