Newman is on trial in the Perth District Court accused of causing grievous bodily harm to Brett Marris, then aged 20, in November last year.
Prosecutors alleged on Monday that Newman had a grudge against Mr Marris because he had slept with Newman's girlfriend when the couple were on a break.
After weeks of threats from Newman on Facebook, the pair saw each other at the Library Nightclub in Northbridge, where they fought for about 10 seconds before bouncers broke up the fight, the court heard.
But the defence team said Newman, who was the only one of the pair sober that night, acted in self-defence against a taller man. Mr Marris testified that Newman confronted him near the entrance of the club and wanted to have a fight outside.
He said Newman was loud and aggressive, and took the first swing.
Mr Marris said he drank 10 mixed drinks that night and had pushed Newman with an open hand, but denied placing his hand on Newman's neck and upper chest.
"He had been threatening me for months prior over Facebook," he said.
In one message shown to the court, Newman wrote: "Start digging a grave."
Newman also called Mr Marris a "little girl" and asked why he did not want to meet "for a crack".
In another message, Newman wrote: "I won't rest til you ey (sic) hurt."
Mr Marris said he did not seek medical treatment immediately after the fight, but applied ice to his jaw.
He said he was in more pain the next day but it was not until the following day that he went to a hospital and had surgery. Mr Marris also admitted he wanted to get compensation.
West Coast captain Darren Glass was sitting in court. The trial continues.
