Vincent Tabak claims he throttled Joanna Yeates in just 20 seconds after failing to stop her screams, a court heard today.
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".
Tabak sat in the dock with his hands covering his face as Mr Reardon told how he had realised Miss Yeates was missing after returning Sheffield to their flat in Clifton, Bristol.
He told jurors he paced around the flat, tidying as he went, and trying to piece together what may have happened.
Mr Reardon had returned from visiting relatives in Sheffield to an empty flat on Sunday December 19.
He said he had repeatedly called and texted her over the weekend after leaving Miss Yeates with a "hug and a kiss" on the Friday.
"I was quite annoyed that I had not been told what her plans were and she had not got back to me and I was starting to feel quite worried," he told the jury.
Worry turned to panic after he found her mobile phone and then keys.
Mr Reardon told jurors: "I started pacing around the flat. I had this increasing buzzing level of stress.
"I didn't know what was going on, so I walked around the flat and, trying to find out what she might have done, so I went tidying up as I went, trying to find out what she was wearing by looking at what was out.
"I panicked really. It was midnight. It was a realisation that now something is wrong."
The jury was shown CCTV footage of Miss Yeates, who had been off work with a cold the previous day, wearing her white ski jacket as she walked with Mr Reardon through the snow on the morning of December 17.
The pathologist who inspected her body was questioned about Tabak's evidence as he was re-examined by the prosecution.
Mr Lickley asked Dr Russell Delaney about Tabak's statement that he held Miss Yeates's throat "for about 20 seconds".
Dr Delaney replied: "That period of time would be sufficient to cause the signs of venous obstruction and would be long enough to result in her death."
Tabak's QC William Clegg suggested his client used just one hand to strangle her.
Miss Yeates is said to have suffered 43 injuries at the hands of Tabak on December 17.
Her body was found "in a foetal-type position" on Christmas Day by dog walker Daniel Birch.
Mr Birch and his wife, Rebecca, spotted the pocket of her denim jeans exposed through the snow as they walked their chocolate Labrador Roxy along Longwood Lane, Failand, Somerset.
The case was adjourned until tomorrow when the court will hear from more prosecution witnesses.