Amanda Knox's appeal against her 26-year sentence for the murder of Meredith Kercher received a massive boost on Saturday when a judge granted her request for a comprehensive review of key DNA evidence that was used to convict her and her former boyfriend Raffaele Sollecito.

After two appeal hearings in Perugia, judge Claudio Pratillo Hellman said he was convinced that the complexity of the case merited a review of forensic tests in the name of "reasonable doubt". Knox, 23, and Sollecito, 25, were convicted of killing Kercher, a British exchange student, in Perugia, in November 2007.

Hearing the news, Knox began to take deep breaths, her mother and stepfather both began to cry and Sollecito beamed broadly at his family.

"Amanda is stunned," said her mother, Edda Mellas, after conferring with her daughter. "She is in shock but happy – this brings hope."

Knox's college friend Madison Paxton, who has moved to Perugia to be near to Knox, said that she was "full of adrenaline" after Hellman said he was appointing experts to re-analyse a knife and a bra clasp considered crucial to the convictions. "This is an indication that they will look at this with fresh eyes," said Paxton.

The two defence teams had asked for experts to complete an independent review of the analysis of the knife, found in Sollecito's kitchen drawer and considered a murder weapon. It contained tiny DNA traces of both Knox and Kercher, according to police, but insufficient quantities to be reliable, said the defence.

"If it is not possible to check the identity of the DNA, we will check on the reliability of the original tests," said Hellman in his ruling, which came after the judge and jury retired to deliberate for an hour and 20 minutes.

Also disputed is the discovery of Sollecito's DNA on Kercher's torn bra clasp, found in her room 49 days after her death, during which time it had been moved accidentally by investigators.

Hellman agreed to review the bra clasp evidence after defence lawyers claimed it had been contaminated during the crime scene investigation. "It would have been very strange for a guilty party to insist so much on these tests," said Francesco Sollecito, the accused's father.

Hellman said that he would appoint two independent experts from the University of Rome to oversee the tests and would set a timetable on 15 January. A specialist in civil law, Hellman is running the Knox appeal almost by accident since fellow judges who might have been appointed were either transferred or had retired.

Chris Mellas, Knox's stepfather, said that Hellman's lack of experience may be an advantage for her. "Because it is not his typical arena, he might be a little less desensitised. The court's summary of the case, which was read out last week, contained many of our points, and that was already something."

Hellman also accepted the defence request to hear from Antonio Curatolo, a homeless man from the area who they say gave mistaken testimony. Curatolo says he saw Knox and Sollecito loitering near the house Knox shared with Kercher on the night of the murder, but the defence lawyers now claim that he had confused the dates.

At the start of the appeal hearing, Knox entered the court looking pale, watery eyed and hunched, while Sollecito, wearing winter boots and his short hair unkempt, smiled in a dazed fashion. After the trial started late, as a result of snowbound roads around Perugia, prosecutor Giancarlo Costagliola said that a review would be "useless" and that "this court has all the elements to be able to come to a decision".

Manuela Comodi, the prosecutor who built the case against Knox with fellow magistrate Giuliano Mignini, returned to the court to try, in vain, to demolish the defence's case for a review.

Knox's prospects were clouded by the court's decision to incorporate into the appeal all documents, evidence and findings from the murder trial of Rudy Guede, who chose a fast-track trial and was definitively sentenced to 16 years on Thursday for his part in the murder following the two appeals he was allowed under Italian law.

By upholding Guede's sentence, the appeal court also gave credence to the arguments made for his guilt at his first appeal, which clearly placed Knox and Sollecito at the scene of the crime with Guede. Mignani, the magistrate who secured the conviction of Knox, warned that the Guede conviction could doom Knox's appeal.

"Guede's sentence states that Knox and Sollecito were involved and it is very rare that one sentence would contradict a definitive sentence."