"I'm happy to say that she's holding her own," Dr. Michael Lemole, the head of neurosurgery at University Medical Center in Tucson, Ariz., said at a news conference.
Giffords, who suffered a gunshot wound to the head on Saturday, can still follow simple commands and has not shown signs of more brain swelling, Lemole said, adding that doctors have been able to ease up on her sedation.
"In fact, she's able to generate her own breaths," Lemole said. "She's breathing on her own."
Still, Lemole said, Giffords' recovery will come at her own pace.
"A penetrating injury through the skull, really the survival, let alone recovery, is abysmal," Lemole said. "She has no right to look this good and she does. We're hopeful, but I do want to underscore the seriousness of this injury and the fact that we all have to be extremely patient."
Earlier, he told NBC's "Today" show that her overnight CAT scan showed no additional swelling.
He cautioned that some patients have the greatest amount of swelling later than the third day after surgery. "We might have to be on pins and needles" for a few more days, Lemole said on CBS' "The Early Show."
Giffords, 40, was shot in the left side of the brain when a gunman opened fire outside a Tucson supermarket where the three-term Democrat was meeting with constituents. During surgery, doctors removed a section of Giffords' skull to ease pressure that could contribute to brain swelling. Since surgery, Giffords has been able to follow simple commands when doctors bring her out of sedation.
She's in critical condition, but she has been pulling at the breathing tube, an encouraging sign to her doctors.
"It implies a purposeful level of consciousness," Lemole told NBC. "So if I were to irritate you or pinch you, it's a natural response to reach up and slap my hand. And the fact that she's able to register that discomfort and then react to it, again it means the brain is working on a higher level."
Doctors say it's too early to say what, if any, long-term problems she'll suffer, but Lemole says anything is possible.
Her family is with her, which Lemole said will become increasingly important when Giffords can communicate more directly once the breathing tube is removed.
"I would not discount the importance of having them at her bedside and just having her being able to hear their voices" right now, he said on CBS.
He said the other shooting victims at the hospital are doing well.
Giffords is the only patient in critical condition. Three are in serious condition and two are in fair condition. Several patients have been discharged.