Oscar Pistorius is expected to take the stand on Friday to testify how and why he shot dead his girlfriend, as defence lawyers open their case in the star Paralympian’s murder trial.

As the only surviving witness to the events inside his house in the early hours of February 14, 2013, Pistorius has the strongest word in his defence that he was in a loving relationship with Reeva Steenkamp and that her death was a tragic accident.

The 27-year-old has to explain why he fired four shots at the model and aspiring television actress through a locked toilet door, and undermine prosecutors’ argument her death was premeditated murder.

It will be the first time he speaks in public since the Valentine’s Day killing, besides pleading “not guilty, milady”, and the occasional “yes, milady” to Judge Thokozile Masipa during his trial.

Defence lawyer Barry Roux has read two almost identical affidavits on behalf of his client – one at his bail hearing last year, another at the start of the trial on March 3.

Pistorius does not have to testify, but his defence team has indicated it believes it will be in his interest to explain the night’s events. If the accused does take the stand, law dictates he has to be the defence’s first witness.

“I think it’s going to be a very tough time for Pistorius. Every person who testifies in court goes through pressure, especially the accused,” said William Booth, a Cape Town-based criminal lawyer not involved with the case.

“Every word could count against you.”

Pistorius’s lawyers would want him to face prosecutor Gerrie Nel’s cross-examination for as little time as possible, but he could spend as much as two days on the stand.

On Tuesday, the state called the last of its 21 witnesses against the athletics star. The court heard testimony that depicted Pistorius as hot-headed and gun obsessed, as well as hearing from police ballistics and mobile phone experts, and a forensic pathologist.

One of Pistorius’s lawyers then said his client would probably take the stand when the defence opened their case on Friday.

“If you don’t testify the court will say, well, they haven’t any version of yours tested under cross-examination,” said Booth.

“His evidence only has to be reasonably possibly true, and he doesn’t have to prove anything.

“If there was somebody who witnessed that, he wouldn’t need to testify. But he’s the only witness.”

Neighbours say they heard screams from a man and woman, and gunshots the night of the killing, while a police ballistics expert said Steenkamp was hit in the hip first before suffering gunshot wounds to her arm, hand and head.

This means she almost certainly cried out in pain – alerting her boyfriend she was in the bathroom – before the lethal shot, prosecutors contend.

They have also asked questions about food in the 29-year-old model’s stomach that suggests she was awake two hours before her death – contradicting Pistorius’s version that they had been asleep.

The defence say the shots followed too close on one another to leave her any time to scream, and that the shrieks were actually Pistorius, who sounds like a woman when he is anxious.

The double amputee, who was born without calf bones and garnered worldwide fame by running on two carbon fibre blades, says he shot Steenkamp because he mistook her for an intruder.

Defence lawyers say the pair were deeply in love, and that most of their mobile phone correspondence was affectionate, despite one message in which Steenkamp said she was sometimes afraid of Pistorius.