Judge Thokozile Masipa ordered an adjournment at the Oscar Pistorius trial and said she would rule on Mr Nel's request tomorrow morning.
The prosecutor questioned why the defence asked psychiatrist Merryll Vorster to testify on behalf of the Olympic runner.
He has suggested that the trial is not going well for Pistorius and his lawyers are floating the idea that a disorder contributed to his girlfriend Reeva Steenkamp’s shooting and therefore Pistorius bears less responsibility for her death.
Nel says he's not arguing #OscarPistorius has mental illness - and never will. But argues court needs clarity on this issue.
— andrew harding (@BBCAndrewH) May 13, 2014
In asking for another professional evaluation of Pistorius's mental state, Mr Nel appeared to be trying to ward off any attempt by the defence to say Pistorius should be treated favourably by the court because of a mental condition linked to his disability.
Pistorius's chief lawyer Barry Roux said at the start of defence-led evidence that the double amputee's vulnerability and disability was at the centre of his case of a mistaken killing.
He said Pistorius should not be sent for 30 days of psychiatric evaluation and that he wanted to call another witness to continue evidence.
Ms Vorster met Pistorius this month, prompting the prosecutor to question whether the timing of her late entry to the stable of witnesses signified a change in tactics by the defence, which has said Pistorius fired out of fear that he was about to be attacked.
PA