Standing in her bathrobe in her Manhattan apartment one recent afternoon, Robin Roberts stares into a hall mirror and barely recognizes the reflection staring back. The athlete's body she has meticulously kept in shape since her days as a college basketball star is 10 lbs. lighter and her once-tight abs are now bloated from chemo. Her short layered hair is gone and her usually bright eyes are clouded with tears. As she sets her alarm for 3:45 a.m., Roberts questions if, come morning, she'll be able to smile into the Good Morning America cameras and pull off the playful banter for which she and her co-anchor Diane Sawyer are famous.

But when the new day arrives, the veteran broadcaster has pushed aside the weepy cancer patient whom one of her sisters calls Chemo Sobby. "I woke up," says Roberts, 47, "and I was really anxious to come in."

She could choose to stay home, of course, and not one of her millions of fans would hold it against her. Ever since July 31, when Roberts announced on air that she had been diagnosed with breast cancer, she's been bombarded with nothing but goodwill from viewers and colleagues. But Roberts wants to work—or maybe she has to work. "You want to say, 'Everything's the same; I'm living with cancer and it's not going to stop me,'" she says. "But until you really test yourself and challenge yourself, I don't think you quite know."

People who know Roberts will beg to differ. "She is such a competitor," says her sister Dorothy Roberts McEwen, 51. "I think she is looking at this like, 'Oh yeah, Cancer, you think you're going to beat me? Oh, I don't think so.'" At work, where Roberts's bosses are letting her decide how much to share with viewers, GMA senior executive producer Jim Murphy says, "I actually think Robin has become even better at her work. She's even more passionate and engaged."

On this morning as she rides to the ABC studios in Times Square, Roberts feels the old Robin resurge. After exchanging hugs with her makeup artist and hair stylist and popping in to say hi to Sawyer, Roberts settles into a chair to let Team Beauty work its magic. As they adjust her wig and apply makeup, Roberts says, "I feel like Humpty Dumpty. Piece by piece, they're putting me back together again." More important than how she looks is how she feels during the morning broadcast. "From 7 a.m. to 9 a.m.," she says, "I am the Robin that most people know and the Robin that I know."

That Robin initially had no intention of telling the world about her cancer. She shared her July 17 diagnosis (the day is marked with a big letter C on her calendar) of DCIS—ductal carcinoma in situ, a noninvasive form of breast cancer—with family and only a handful of friends and colleagues. Beyond that, she says, "I was, like, 'There is no way in God's green earth that I am letting anybody know.'" But viewers sensed a change in her. "Robin seems different since she got back from vacation," one person wrote on GMA's online message board. In the end it was Roberts's 83-year-old mother, Lucimarian, who persuaded her to speak up. "My mother said, 'You probably could help a lot of people if you did this.' When your mama says that to you, in a very gentle way, well, that was the X factor that tipped the scales for me."

The live announcement was agonizing. "That was me at my absolute rawest," Roberts says. "I couldn't have done it without Diane." Sawyer, a close friend, remembers the emotional exchange differently: "Our understanding was, since I rarely tear up, that I would be the one to lead into the piece. Of course, I started the intro and started tearing up and completely failed in my responsibility. She was the strong one."

Roberts's candor touched off an outpouring of heartfelt e-mails, prayers and gifts. But Roberts's fear remained, as did an irrational embarrassment about being sick, spawned by her self-image as an athlete and former ESPN sportscaster. "I felt this kind of shame, that I did something wrong," she says. The night before her Aug. 3 lumpectomy, Dorothy talked sense into her baby sister. "She said, 'When someone tells you they have cancer, do you think they did anything wrong?'" Roberts recalls. "I was, like, 'No,' and she says, 'Well, give yourself a break.'"

The day of the surgery, Roberts's mother and two sisters were on hand at New York City's Lenox Hill Hospital. (Her father, Lawrence, a retired Air Force colonel, died in 2004.) As Roberts was rolled off to have a marble-size tumor removed from her right breast, she believed that if the cancer had not spread to her lymph nodes, "the doctor would remove the lump, then I would have a little radiation, pop some pills and be on my way." She had given her family strict instructions: "When I wake up, I want a thumbs-up sign if it hasn't spread and a thumbs-down sign if it has." When she opened her eyes, Roberts saw her mother, her siblings—and Diane Sawyer—flashing thumbs-ups. Laughing, she says, "I remember thinking, I didn't know we had a blonde family member."

But a few days later, after Roberts's brother dropped her at a CVS drugstore, she received an unexpected—and alarming—call. Examining her 2-cm. tumor under a microscope, doctors discovered that Roberts's cancer wasn't DCIS; it was an invasive carcinoma. Though there was no indication that the cancer had spread beyond the breast tissue, the cells looked aggressive. Chemotherapy was strongly recommended. Says her oncologist Dr. Ruth Oratz: "It's like that fire in your fireplace: You know there are a couple of embers in there, that if you poke them you could start up a fire and burn down the whole house. But if you throw water into the fireplace, you put out all those embers. That's what chemotherapy does."

Accustomed to working out five times a week—Pilates, biking, cardio, light weights—Roberts says she finds the idea of putting poison in her body "hard to wrap my mind around." On a chilly morning, she arrives at her doctor's office for her third chemo session flaunting a button that reads, "Cancer Sucks." Roberts particularly loathes Adriamycin, and when the nurse injects the drug, says Roberts, "I have to look away. If I look at it, I swear, I would just grab the syringe and smash it with my bare hands."

When Roberts's hair began to fall out 17 days after her first chemo treatment, she shaved her head. "I cannot believe how fine I am with being bald," says Roberts. One recent Saturday night as KJ, her Jack Russell terrier, raced around her apartment, Roberts, who's single, danced to Motown with friends. When someone began snapping digital photos, she said, "Hey, send this out to our friends. I want them to see me laughing and smiling and dancing.'"

Still, on air, she keeps the wig. "Would I ever go on without it?" she asks. "I would never do it for effect. It would have to have some purpose." Like the day Sawyer's perfume made her so nauseous that during a commercial break she said, "Diane, I love you, but you've got to help me here." Back on air, Roberts initiated a chemo courtesy discussion, stressing that cancer patients have to speak up so people understand their needs.

With Roberts now halfway through her eight chemo treatments, to be followed by up to six weeks of radiation, Dr. Oratz says,"Robin's prognosis is very good." But Roberts doesn't pretend it's easy. On achy mornings, she says, "I'm just so excited because I had the strength to take a shower. Like, Woo-hoo!" There have been mental adjustments, too. Once devoted to her month-at-a-glance planner, she says, "now I don't put things in the boxes anymore. I'm just focusing on right now." And sometimes that strategy provides all the motivation she needs. On a recent reporting trip to the Middle East, Roberts realized she was working as hard as ever—and that everyone around her expected no less. That's when it hit her. "I've still got it," she says. "I refuse to lose."