PRETORIA (AFP) - Oscar Pistorius' defence lawyer accused a key witness of doctoring evidence with his wife to "incriminate" the athlete on Thursday, as he tried to scupper the prosecution's principal line of attack.

Mr Barry Roux, continuing a tough cross-examination for a fourth day, sought to show that Mr Charl Johnson's first account of events the night Pistorius's girlfriend Reeva Steenkamp was shot dead differed from his sworn testimony.

"There is a design on your side to incriminate," Mr Roux said, claiming Mr Johnson's evidence was made in collusion with that of his wife and fellow key witness, Mrs Michelle Burger.

"You desperately want her version not to form part of this document," said Mr Roux, pointing at the witness.

"You want to extricate any suggestion that this version was also your wife's version."

Mr Johnson, a soft-spoken IT manager wearing a red tie and a navy blue suit jacket, denied Mr Roux's accusation.

"My lady, I dispute that," he said addressing judge Thokozile Masipa.

Mr Roux contends the married couple's testimony has "striking similarities" and is therefore contaminated.

The allegation could lessen the impact of the pair's statements.

In vivid testimony that cast doubt on the Paralympian's claims of a "tragic accident," the pair have told the court they heard a screams then gunshots on Valentine's Day 2013 at Pistorius's home.

The couple's account would undermine Pistorius's claim that he shot his girlfriend, a 29-year-old model and law graduate, through a locked toilet door after mistaking her for an intruder.

OSCAR 'SCREAMS LIKE A WOMAN'

Mr Roux struggled to get Mr Johnson to admit that what he heard that night may have been Pistorius's screams, not those of a woman.

Mr Johnson said he was sure of what he heard.

"I'm convinced that I heard a lady scream and it's easy to distinguish a lady voice from the male voice," he said.

He also rejected the Mr Roux's claim that he may have heard a cricket bat hitting a door rather than gunshots.

"I do not believe you could strike a door so rapidly with a cricket bat."

Earlier in the morning, Mr Roux apologised to Johnson for reading out loud his phone number in court.

Mr Johnson told the court Wednesday that his "privacy has been compromised severely" and he had received threatening phone calls and messages on his phone.

"I hope that Mr. Nel yesterday relayed my apology," said Mr Roux, referring to the state's lead prosecution lawyer.

"I did not at that point realise the consequences."

Pistorius, a double amputee known as the "Blade Runner" for his carbon-fibre running blades, faces 25 years in South Africa's brutal jails and an abrupt end to his glittering sporting career if convicted.

Thursday was the fourth day of a trial which is expected to last three weeks, but could drag on much longer.

During a recess on Thursday there were emotional scenes as Pistorius's sister Aimee approached one of Steenkamp's friends and family - who was sporting a picture of her on the lapel.

A tearful Aimee Pistorius sat and talked to the woman for a few minutes before going back to embrace her other brother Carl Pistorius.