Navratilova, 53, told ABC's "Good Morning America" the diagnosis was made in February during a routine mammogram. She said her doctors determined after a lumpectomy the cancer had not spread to her lymph nodes.
The Czech native who now lives in Florida and is a spokeswoman for AARP, is to receive six weeks of radiation treatments in May. She said she expects to "make a full recovery" with only a "very small chance" the cancer will return.
In an exclusive interview with People magazine, Navratilova said she cried when she was told she had cancer in her left breast.
"It knocked me on my ass, really," People quoted her as saying. "I feel so in control of my life and my body, and then this comes, and it's completely out of my hands."
The former world No. 1 won a record 167 singles titles during her pro career, starting in 1974 and ending in 1994. They include 18 Grand Slam singles crowns, including nine at Wimbledon.
She also won 31 Grand Slam doubles titles and 10 mixed-doubles championships, the final one at the 2006 U.S. Open.
People said she was diagnosed with ductal carcinoma in situ, or DCIS, a non-invasive form of breast cancer.
Navrotilova told People she had gone four years between mammograms.
"I let it slide," she said. "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."