Amanda Knox, 26, who is fighting to clear her name after being re-convicted of the killing of Meredith Kercher, 21, in the Italian university town of Perugia was also the one who wielded the knife, the Florence court said.
The new revelations came in a document explaining February's guilty verdicts handed down to Knox and her former boyfriend Raffaele Sollecito for the murder.
The pair had spent four years in an Italian jail after a court found that they had murdered Ms Kercher in 2007.
But the convictions were overturned on appeal and Knox returned home to Seattle in 2011. Sollecito is also partly at liberty. Both were found guilty again after a re-trial earlier this year.
In the new report, the Florence court described the sex game theory initially used to convict the pair as not plausible.
It was not "credible that the four young people began a group sex activity that Meredith Kercher later suddenly no longer wanted to pursue further," the court said. "This hypothesis is not compatible with the personality of the English girl."
The report said the Kercher and Knox did not have a good relationship and had an argument about flatmate issues on an evening when Knox and Sollecito had taken drugs, which then escalated, it said.
Knox, Sollecito and Ivory Coast-born Rudy Guede, who is serving a 16-year sentence for his part in the murder, attacked Kercher together and pushed her into her bedroom, the court said.
Guede assaulted her to "satisfy his sexual instinct" while the two others participated "in a desire to abuse and humiliate the English girl."
The three then killed Kercher so she could not report the sexual assault, the court said.
A knife discovered in Sollecito's house "was one of the two weapons used in the murder, and was the one that was held by Amanda Marie Knox, who wounded Meredith Kercher on the left part of her neck, causing the only fatal wound," it said.
Knox and Sollecito, who is free on bail but is not allowed to leave Italy, are fighting their convictions in a final appeal process expected to conclude in 2015.
Knox and Sollecito continue to proclaim their innocence and the American student said she will not return willingly to Italy.