Kerstin Fritzl, at 19 the oldest of the seven children Fritzl fathered by raping his daughter Elisabeth, had been kept unconscious by doctors treating her for a mystery infection.
But they woke her on Sunday for a touching reunion with her mother, brothers and sisters. At one point doctors feared for her life but said on Wednesday she is now expected to make a full recovery.
Kerstin was one of the three children that Fritzl kept imprisoned along with their mother in a dungeon he built beneath his home in the town of Amstetten. Three others were raised normally by Fritzl and his wife in the house. The seventh child died in infancy.
Kerstin was taken to hospital on April 19, unconscious and suffering from multiple organ failure. Her arrival raised questions about her background that eventually led to the unmasking of the horror that Fritzl began 24 years ago.
Albert Reiter, the doctor who treated Kerstin, said she greeted him after being awoken during a morning visit.
"I say to Kerstin -'Hello Kerstin' - and Kerstin tells me: 'Hello,'" Dr Reiter said.
"For all of us, Kerstin's surprising recovery is a great relief," said Berthold Kepplinger, director of a psychiatric clinic where the reunited family is recovering from their ordeal.
Dr Kepplinger, who described the reunion as a "touching moment," said Kerstin has expressed "many wishes," including going on a boat ride.
Last week a court decided to hold 73-year-old Fritzl in custody for for another two months.