US student Amanda Knox's second appeal trial in the murder of her British roommate opened on Monday, but the star defendant was absent.

Italy's highest court ordered in March a new trial for Knox and her Italian ex-boyfriend, Raffaele Sollecito, overturning their acquittals in the gruesome 2007 slaying of Meredith Kercher. The Court of Cassation gave a harsh assessment of an appeals court acquittal in 2011, saying it was full of "deficiencies, contradictions and illogical" conclusions.

During opening statements, lawyers for Knox and Sollecito requested an array of new expert opinions and evidence to reach a definitive verdict, including examining the handle of the knife that was the purported murder weapon and the handling of the crime scene.

Knox defense lawyer Carlo Dalla Vedova said there was a risk of an "infinite trial", since the charge of murder has no statute of limitations.

Sollecito's lawyer Giulia Bongiorno asked the court to accept only "reliable evidence", saying the intense media attention on the case had affected the three previous trials.

The appellate court in Florence is expected to re-examine forensic evidence to determine whether Knox and her ex-boyfriend helped kill the 21-year-old Kercher while the two women shared an apartment in the Umbrian university town of Perugia. The prosecution advanced the theory that Kercher died during a sex-fueled game gone bad.

Knox, now a 26-year-old University of Washington student in Seattle, has not returned to Italy for the trial, nor is she compelled by law to do so. The appellate court noted the absence both of Knox and Sollecito, but did not declare either in contempt.

"We refute the idea that because Amanda is not coming, that Amanda is guilty, that Amanda is using a strategy. Amanda always said she was a friend of Meredith's. Amanda has always respected the Italian justice system," one of Knox's defense lawyers, Luciano Ghirga, told reporters before the trial opened.

Knox and Sollecito, now 29, were convicted and later acquitted in Kercher's death. Knox served four years of a 26-year sentence, including three years on a slander conviction for falsely accusing a Perugia bar owner in the murder, before leaving Italy a free woman after her 2011 acquittal.