The judge hearing the case against the double-amputee runner called an early adjournment yesterday after Pistorius began crying while testifying about how he smashed a toilet door after shooting four times through it, and found Ms Steenkamp inside.
He wailed loudly, his head in his hands and went out of a side door of the courtroom.
Pistorius denies murdering the 29-year-old law graduate and model, saying that he shot Ms Steenkamp after mistaking her for an intruder.
Prosecutors say he killed her after an argument.
The Olympian is charged with premeditated murder over Ms Steenkamp's death and faces a life sentence, with a minimum of 25 years before parole if convicted.
At one point during yesterday's proceedings, the South African athlete changed out of his suit and returned wearing a white long-sleeve T-shirt and shorts, showing his prosthetic legs to show how he stood by the toilet door which he fired into last year.
The bullet-riddled toilet door was set up in the courtroom in Pretoria to show how the shooting took place.
Pistorius claims he was not wearing his prosthetics - and therefore felt more vulnerable - when he shot through the door, killing Ms Steenkamp.
In the courtroom, Pistorius was asked by defence lawyer Barry Roux to stand by the bullet-marked toilet door, which has been in the courtroom as an exhibit for much of the trial, while wearing his prosthetics.
Pistorius then sat on a wooden bench and took off his prosthetics before again approaching the wooden door to show how tall he was next to it while standing on his stumps.
Pistorius, 27, told the court how he heard a window sliding open in his bathroom in the middle of the night on Valentine's Day last year.
He said he was convinced an intruder was breaking in and that he needed to arm himself.
"That's the moment that everything changed," he said, his voice tense with emotion.
"I thought that there was a burglar that was gaining entry to my home.
"I wanted to put myself between the person who gained access to my house and Reeva."
Grabbing a 9 mm pistol lying under the bed, he moved on his stumps down the passageway leading from the bedroom towards the bathroom and the would-be intruders, he testified.
According to his account, as he peered round the door of the bathroom, his right arm holding the 9mm outstretched, his left steadying himself against the wall, he noticed the bathroom window was open, confirming his worst fears.
"I wasn't sure where to point the firearm," he said.
"My eyes were going between the windows and the toilet. I stood for some time. I just stayed where I was and kept on screaming.
"Then I heard a noise from inside the toilet that I perceived to be somebody coming out of the toilet.
"Before I knew it, I had fired four shots at the door."
He said he continued to shout at Steenkamp to call the police, but slowly started to realise that she was not responding and might have been the one behind the door.
As he described to the court bashing in a door panel to gain access to the toilet, he broke down, sobbing uncontrollably in the witness stand, leading judge Thokozile Masipa to adjourn the hearing for the day.
Pistorius first took the witness stand on Monday, making a tearful apology to Ms Steenkamp's family for killing her as Ms Steenkamp's mother, June, sat in the courtroom, staring at him without expression.