Vincent Tabak will tell a jury for the first time in his own words how he strangled Joanna Yeates.
The 33-year-old, who admits manslaughter but denies murder, will give his own version of events in a bid to explain claims he did not intend to kill her.
Amid beefed-up security at Bristol Crown Court, Tabak is expected to say her death was "pure chance".
Tabak, who has kept his head in his hands for the previous eight days of evidence, claims he misread his next-door neighbour's signals after she invited him in for a drink. He says he put his hand to the 25-year-old's throat after she protested at his advances.
Defence QC William Clegg said Miss Yeates's death was tragic misfortune. Tabak "completely misread the situation" before "Joanna went limp", Mr Clegg claimed. "It was pure chance that Vincent Tabak and Joanna Yeates ever met on December 17 last year," the barrister told jurors on Wednesday.
Miss Yeates is said to have invited Tabak into her flat after smiling at him as he walked past her kitchen window, Mr Clegg said. Mr Clegg said Miss Yeates and Tabak were virtual strangers - but were both at loose ends at their flats in Clifton.
"If Joanna Yeates had stayed for one more drink in the Ram pub, she'd be alive today," the barrister said. "If Vincent Tabak had left half an hour earlier to go to Asda, as was his intention, he wouldn't be standing in the dock now."
Mr Clegg has said he would not try to justify Tabak's actions after her death, saying his client was "living a lie" by attending dinner parties and attempting to carry on his life as normal.
He said it was "frankly disgusting" that Tabak had tried to hide the body and "did everything he could to cover his tracks". But he urged the jury to focus on what happened in Miss Yeates's flat as Tabak takes to the stand.
Mr Clegg said: "What he is being tried for is whether when he killed Joanna Yeates that was planned, premeditated and something that he intended to do, intended to kill or cause really serious harm to her, or whether when he was responsible for her death, when he killed her, he panicked and did it without thinking of the consequences, without intending that she should die, without intending that she should suffer really serious harm. That's the issue you need to focus on."