Pistorius, who had his lower legs amputated as a baby, is accused of murdering his law graduate and model girlfriend Reeva Steenkamp at his home in Pretoria on Valentine's Day last year. If convicted, he could face life in prison.
Since the trial opened in early March, Nel has portrayed Pistorius as a gun-obsessed hot-head who shot 29-year-old Steenkamp four times through a locked toilet door where she was taking refuge after a heated argument.
The defence team says the track star was a vulnerable and caring boyfriend who killed Steenkamp in a tragic accident after mistaking her for an intruder hiding behind the door.
"We will argue that the accused's version should be rejected," Nel told the Pretoria court during his closing arguments.
"If the accused version is rejected it means my lady that there was no perceived intruder whatsoever."
Pistorius sat impassively through the opening minutes of the proceedings, which are expected to last two days. After that, judge Thokozile Masipa, who has more than 4,000 pages of evidence to review, will retire to consider her verdict.