Knox, an American student, and Rudy Guede were convicted in the 2007 death of Meredith Kercher in the house the two shared in Perugia
Guede was held in the same prison as Mario Alessi, Sunday's witness.
It was during that time that Guede allegedly told Alessi a friend killed Kercher after she refused to follow through with a previously arranged sexual encounter.
Knox's stepfather said he wishes Alessi's testimony was heard sooner.
"When a person comes forward and says that they have testimony as a witness, the court is supposed to hear them and the previous court did not," he said. "As to the veracity of what he is saying, I don't know. Nobody knows because they would not investigate his claim."
Knox was sentenced to 26 years in prison.
Guede has denied Alessi's claim.