She said he treated his daughter as his property - raping, imprisoning and abusing her.
Fritzl's lawyer, Rudolf Mayer, said his change of plea to guilty on Wednesday had been made in an emotional state.
He also confirmed that his client's daughter had been in court watching proceedings earlier this week.
Eva Plaz, the lawyer for Fritzl's daughter, told the jury they could either believe her client or Josef Fritzl.
She said Fritzl had always known he was guilty of the baby's death.
Fritzl's guilty pleas could be a mitigating factor, but only at the discretion of the judges and jury.
He could face life imprisonment for murder, 20 years for enslavement and up to 15 years on other charges.
Dressed in a grey jacket and blue shirt and tie, Fritzl, who was ringed by police, walked to the courtroom on Thursday with his face uncovered, having concealed his face with a binder in front of the cameras for the first two days of his trial.
Fritzl's psychiatrist Dr Heidi Kastner speaks to the BBC's Newsnight programme
The Austrian imprisoned his daughter in a cellar under his house for 24 years and repeatedly raped her.
The daughter and three of the children fathered by Fritzl were kept captive in the cellar until the case came to light in April last year, when one of the children became seriously ill and was taken to hospital.
In his surprise confession on Wednesday, Fritzl admitted murdering by neglect one of newborn twin boys his daughter gave birth to in 1996, having failed to arrange medical care for the ailing infant.
The other three children were raised in the family home by Fritzl and his wife, after he told people that his daughter had abandoned them and joined a sect.
The daughter and her six surviving children been recovering from their ordeal in a psychiatric clinic and at a secret location.
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