"I am tired. It's not going to change," Pistorius said.
"You're trying to cover up for lies," Nel said, "and I'm not convinced."
Judge Thokozile Masipa then asked Pistorius if he was too tired to continue testifying, saying: "You can be at a disadvantage when you're in that box."
Pistorius kept going.
Later, Masipa tried to rein in the prosecutor, telling him: "Mind your language. We don't call a witness a liar, not while he's in the witness box."
The prosecutor has seized on virtually every opportunity to challenge the star athlete's credibility, asserting that he had a string of unlikely excuses for why he wasn't to blame in the gun charges he faces on top of murder. In casting doubt on the Olympian's honesty, Nel is pushing the prosecution's argument that Pistorius is also lying that he killed Steenkamp by mistake.
On Friday, Nel argued that Pistorius lied about an incident years ago — when he says someone shot at him from another car on a highway — to build a backstory that he had a long-held fear of being attacked.
Pistorius said he saw a "muzzle flash" and heard "a banging noise" as a black Mercedes drove past him, and he turned off the highway and went to a restaurant parking lot and called someone to pick him up. Nel asked for the identity of the caller; Pistorius replied that he couldn't remember.
Pistorius is not allowed to consult with his lawyers during the cross-examination, which resumes Monday.