Rocking back and forth in the dock, Pistorius put his hands over his ears as neighbour and radiologist Johan Stipp recounted how he entered his house to find the distraught Paralympian bent over, attempting to resuscitate his girlfriend.
Stipp noticed a wound on Steenkamp's right thigh, right upper arm, and "blood, hair and what looked like brain tissue intermingled with that" on top of the skull.
Amid the scene of horror, Stipp said he quickly realised the 29-year-old model's injuries were too severe to be survivable.
"She had no pulse in her neck, she had no peripheral pulse, she had no breathing movements that she made. She was clenching down on Oscar's fingers as he was trying to open her airway."
"I opened her right eyelid, the pupil was fixed dilated and the cornea was milky, in other words it was already drying out, so to me it was obvious that she was mortally wounded."
During the testimony sobbing could be heard around the courtroom as Steenkamp's family and friends sat arm in arm, disconsolate.
Stipp continued: "While I was trying to ascertain if she's revivable, Oscar was crying all the time, he prayed to God to please let her live, she must not die."
"He said at one stage, while he was praying, that he will dedicate his life and her life to God if she would just only live and not die that night."
"I couldn't do anything for her, she was way too seriously injured for that."
Stipp also said he wanted to locate Pistorius, fearing the sprinter may have been a danger to himself.
"Oscar was emotionally very, very upset and I didn't know the situation in the house so I thought maybe he was going to hurt himself."
Stipp's evidence is the first eye-witness account of what happened inside Pistorius's home that evening since the trial began on Monday.
Pistorius stands accused of murdering Steenkamp, a 29-year-old model and law graduate.
He claims to have shot her through a locked toilet door after mistaking her for an intruder.
Stipp also testified to being woken by gunshots and screams coming from Pistorius's home on the night of February 14, 2013, and then rushing over to see if he could help.
But the sequence of events appeared not to tally exactly with the evidence from three other neighbours who heard screams then shots.
Pistorius's defence lawyer earlier accused one of those witnesses of tailoring his evidence to fit his wife's account in order to "incriminate" the athlete.
"There is a design on your side to incriminate," advocate Barry Roux said, claiming Charl Johnson's evidence was made in collusion with that of his wife and fellow key witness, Michelle Burger.
"You desperately want her version not to form part of this document," said Roux, pointing at the witness. "You want to extricate any suggestion that this version was also your wife's version."
Johnson, a soft-spoken IT manager wearing a red tie and a navy blue suit jacket, denied Roux's accusation.
"My lady, I dispute that," he said addressing judge Thokozile Masipa.
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.
