Vincent Tabak claims he throttled Joanna Yeates in just 20 seconds after failing to stop her screams, a court has heard.
Part of the killer's account was heard for the first time as her boyfriend described his panic after she went missing.
Greg Reardon told how annoyance at returning to a messy flat turned to "buzzing stress" as he realised she had disappeared.
Mr Reardon appeared in the witness box after prosecutor Nigel Lickley QC read an extract from Tabak's defence statement on September 22.
Mr Lickley said Tabak was facing his 25-year-old victim when the attack took place.
"He put one arm around her back with his hand in the middle of her back," Mr Lickley said.
As Miss Yeates screamed, Tabak - who admits manslaughter but denies murder - says he initially put his other hand over her mouth.
Mr Lickley said: "He removed his hand from her mouth and the screaming continued. He then put his hand around her throat.
"He believes it was the one that had been from behind her back and held it there for about 20 seconds."
As evidence continued into a second week at Bristol Crown Court, the victim's best friend, Rebecca Scott, told how Miss Yeates and Mr Reardon were "the perfect couple".