Travis Gray, right, outside court before giving evidence.

Travis Gray, right, told the court on Tuesday of hearing his friend Luke Adam's head 'crack' on the pavement.

Moments before the single punch that left footballer Luck Adams with severe head injuries, he had turned to a friend and said "let's smash these c....s," the District Court has heard.

Dylan Winter, 20, was charged with inflicting grievous bodily harm on Mr Adams, 21, after the former Swan Districts ruckman was struck to the head in the early hours of May 1, 2011.

Mr Winter has pleaded not guilty, with his defence lawyers claiming he punched Mr Adams – a larger and stronger opponent – in self-defence after being threatened.

Mr Adams remembers buying a bottle of vodka after playing football on April 30, 2011, but he then recalls nothing else – including the encounter with Mr Winter – until he woke up in Royal Perth Hospital weeks later, the court heard on Monday.

Giving evidence on Tuesday, Mr Adams' teammate and friend Travis Gray told the court he was with Mr Adams on Lake Street in Northbridge shortly before 3am on May 1.

He said he was walking a couple of metres ahead and not paying attention to an argument Mr Adams was having with somebody on the other side of the road.

He said he turned back when Mr Adams said "let's smash these c....s".

Moment later he saw Mr Adams "falling backwards with his arms in the air," he told the court.

"It was like he was motionless falling to the ground. I heard his head crack as it hit the ground," Mr Gray said.

Mr Gray said he was punched in the back of the head as he ran to Mr Adams' aid, but did not see by who.

He said a male was standing about a foot from Mr Adams when he started falling but then he fled in a taxi with a group of people.

"I was more in a state of shock, I just kept thinking he was going to wake up," he said.

"I tried to wake him up but he wasn't waking up and then the blood started pooling around his head."

Passers-by came to assist and call an ambulance.

The trial is expected to continue today.