A judge will rule on Wednesday whether Oscar Pistorius will be committed to one month of psychiatric tests to establish if he has a "general anxiety disorder," as his murder trial reaches its climax.

Prosecutor Gerrie Nel formally asked the court to have Pistorius committed Tuesday, after defence psychiatrist Meryll Vorster claimed the sprinter's deep-seated anxiety would have given him a heightened fear of crime.

During two months of trial, Pistorius's lawyers have sought to portray him as manically obsessed with safety after a difficult childhood and in the face of high crime levels in South Africa.

Those factors, they argue, help explain his reaction on Valentine's Day last year when he allegedly believed his girlfriend to be an intruder and shot her dead through a locked toilet door.

Nel said he did not believe Pistorius's mental state was unusual, but warned the athlete's defence team may try to use that evidence to limit sentencing, launch an appeal or reset the case.

"My lady, accused persons have in the past replaced their counsel, particularly when things go wrong," said Nel.

In a hint of his closing arguments, Nel also accused Pistorius of frequently changing his defence and using poor mental health as a fallback.

Pistorius lawyer Barry Roux furiously objected to Nel's request and went on the attack.

"My lady I say it with great deference, Mr Nel's reading of the law is quite unfortunate."

"It's just a ruse to get a second opinion and it's simply going nowhere."

Judge Thokozile Masipa said she would rule on the request on Wednesday.

"I'll be back with my decision tomorrow," she said.

If Pistorius is referred, he will wait until a bed is free at the notoriously backlogged hospitals in South Africa to undergo a 30-day mental evaluation.

If he is not referred, evidence in his trial could conclude as soon as this week.

Not optimal

Psychiatrist Vorster earlier testified that Pistorius had a "general anxiety disorder" that had an impact on his personal and sexual relationships.

"Did he function socially, yes, one would say he did," she said, "but not optimally."

He took on new friends "in order perhaps to avoid being lonely," said Vorster, "he was simply in their company as to not feel alone."

Earlier in the trial, Pistorius's former friend Darren Fresco and ex-girlfriend Samantha Taylor testified for the state, saying the 27-year-old runner shot a gun out of a moving car's sunroof.

When Fresco finished giving his evidence, he took a seat with Steenkamp supporters in the front row of the public gallery.

Vorster, a confident witness who spent a second day on the stand, said Pistorius doesn't fare better in sexual relationships.

"His sexual relationships appear to have been quite short in duration," said Vorster.

Nel argued that Pistorius did not take safety precautions in his house to combat crime, including the absence of burglar bars, a functioning alarm system, and an open bedroom door.

Such negligence did not fit the profile of a person who was abnormally concerned with safety, said Nel.

The star sprinter claims he mistakenly shot girlfriend Steenkamp through a locked toilet door in the early hours of the morning, believing she was an intruder in his upmarket Pretoria home.

When court adjourned, the two lawyers shared an exchange, both breaking into big smiles.

Roux gave Pistorius a deep hug, clasping his neck in a show of support.