* First retrial hearing opens in Florence court
* Judge orders checks on previously untested DNA trace
* Jailed mafia member to be heard in court again
An Italian judge presiding over the retrial of American student Amanda Knox on Monday ordered new DNA tests on the knife that prosecutors say was used to kill her British roommate in 2007.
Knox and her Italian ex-boyfriend Raffaele Sollecito were found guilty in 2009 of murdering 21-year-old Meredith Kercher. They were acquitted on appeal in 2011 but the acquittal was later quashed by Italy's supreme court.
Neither appeared in court on Monday for the first hearing in the retrial. Knox, now back home in Seattle, has said she will not be returning to Italy.
Judge Alessandro Nencini will also hear new testimony from jailed Naples mafia member Luciano Aviello, who previously said his brother killed Kercher. He is due to appear in court on Friday.
The new checks on the presumed murder weapon - a kitchen knife found in Sollecito's house - will examine a trace that was not previously tested because experts said it was too small to produce reliable results.
The court will also assess photographs of Sollecito's nail-bitten fingers which the defence have presented.
The supreme court overturned the acquittal of Knox and Sollecito in March, citing "contradictions and inconsistencies" and paving the way for the retrial.
Kercher was found with more than 40 wounds, including a deep gash in the throat, in the apartment she shared with Knox in Perugia, a picturesque town in the central Umbria region that attracts students from around the world.
Knox, 26, has denied involvement in the killing. She told U.S. television this month that "common sense" told her not to return to Italy. She is not obliged to attend the hearing and can be represented by her lawyers, who said she is watching the retrial closely from home in Seattle.
Sollecito, 29, who has also protested his innocence, plans to attend some of the hearings, his father Francesco said, adding he was confident his son's innocence would be confirmed.
"Deeper examination can only demonstrate what we already know, that Raffaele Sollecito has nothing to do with what that poor girl had to suffer," he told reporters.
The judge rejected requests from the defence team to test semen stains found on a pillow at the crime scene and a re-examination of call logs on Kercher's mobile phones.
Francesco Maresca, a lawyer for the Kercher family, said there was sufficient evidence against Knox and Sollecito and the supreme court's decision to throw out their acquittals had reinforced his view.
"We have always maintained that they are guilty and that they were present at the crime scene," he told reporters.
Knox has said she wants to visit Kercher's grave in England, but the Leeds University student's family said in a statement at the weekend this was Meredith's "safe place" and they hoped "that is respected by all".
Maresca handed the judge a letter from Kercher's family excusing their absence in court on Monday, citing health problems. Her family have welcomed the new trial.
"Nothing will ever bring our beautiful Meredith back ... but we need to know what happened and she at least deserves the dignity of truth," they wrote in the letter.
If found guilty, Knox could appeal again to Italy's supreme court. If that failed, Italy could request her extradition.
When explaining its decision to overturn the acquittals, the supreme court said the appeals hearing had failed to take all the evidence into consideration. It said the one person still in jail for the murder, Ivory Coast-born Rudy Guede, who is serving a 16-year sentence, was unlikely to have committed the crime alone.