"I feel so in control of my life and my body, and then this comes, and it's completely out of my hands," she said.
Doctors detected the disease early, during a routine mammogram, and expect Navratilova to make a full recovery.
"There's only a one-percent chance of anyone with this diagnosis would die of breast cancer," said Dr. Shelley Hwang, a breast surgeon at UC San Francisco.
Navratilova's cancer is confined to the milk ducts and has not spread to the surrounding tissue or the lymph nodes. She had a lumpectomy in February and will undergo six weeks of radiation in May.
"It is just in that one breast," she said on Good Morning America. "I'm okay and I'll make a full recovery."
Navratilova, who was born in Czechoslovakia and moved to America in 1973 to play tennis, told the talk show she was feeling healthy and "absolutely 100 percent fine" when she was diagnosed. She's still struggling to deal with it emotionally, though.
"I'm this healthy person," she said. "I've been healthy all my life, and all of a sudden I have cancer, are you kidding me?"
The nine-time Wimbledon champ is the Health and Fitness Ambassador for the AARP. Two weeks after he lumpectomy, she played in the Hit for Haiti tennis fundraiser. She also participated in a 25-mile bike ride as part of a triathlon relay in Hawaii in March.
She won't be letting radiation treatment slow her down, either. Navratilova will travel to France during her treatment to be a commentator for the Tennis Channel at the French Open. She also plans to climb Mount Kilimanjaro in December and still plays tennis and ice hockey.
The diagnosis came during Navratilova's first mammogram in four years, prompting her to share her story. She hopes that it will inspire other women to get yearly check-ups.
"I let it slide," she told People. "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."
Navratilova will be hosting a live web chat on the AARP website on Thursday, April 8, at 1 p.m.