Andrew Hall said he had been walking behind his friend Karl Poole when he saw him and Mr Tompkinson, 57, rowing.
The actor, who lives in Whitley Bay, North Tyneside, denies inflicting grievous bodily harm.
Mr Hall told Newcastle Crown Court his friend was snoring which he knew could "represent a serious head injury".
The court previously heard Mr Hall and Mr Poole had been drinking vodka and Jägermeister with Red Bull since midnight before going to the beach with a bottle at about 05:30 BST on 30 May 2021.
They passed Mr Tompkinson's house in Beech Grove on their way home.
On Wednesday, jurors were played a recording of the Stockton-born actor calling police to ask for the two drunk men outside his home to be moved on.
Mr Hall said he saw his friend and the actor gesturing at each other and tried to calm the situation by saying he was a social worker.
"Then I heard a hit of flesh and I saw Karl hit the floor," he said.
"He was knocked out and snoring which I know - I'm not a paramedic but I know from training that I have done - I know that it can represent a serious head injury."
Under cross-examination Mr Hall agreed he could not remember parts of what happened - including falling over with Mr Poole when play fighting - because he had been drinking.
He also described the interaction with Mr Tompkinson on the actor's driveway as a "blur".
It was put to Mr Hall his evidence had "dramatically changed" compared to his initial statement to police, in which he said he did not know how his friend ended up on the floor.
Mr Tompkinson's defence barrister, Nicholas Lumley KC, said to him: "You did not hear the sound of the impact that caused him to fall."
Mr Hall replied: "Not 100%."
The court was shown mobile phone footage of Mr Poole lying in the street in just his underpants with Mr Hall crouched beside him.
Mr Lumley KC suggested they were "extremely drunk that night" and became aggressive when Mr Tompkinson suggested calling the police.
"He put his shoulder to you and put his hand out to keep Mr Poole at bay and that's when Mr Poole staggered and fell to the ground," he told Mr Hall.
The court also heard from Mr Tompkinson's neighbour Caroline Davidson who described being woken by "hysterical laughing".
Looking out of her window she saw two very drunk men on the ground by a tree, "wobbling side-to-side" even though they were sitting down, she said.
She told the court she was woken a second time by the "different tone" of another man - Mr Tompkinson - who she knew lived opposite but had not spoken to.
Mrs Davidson said the actor was gesturing for the pair to move away and, at one point, "pulled his fist back" but put it down "more or less straight away" before walking back towards his house while apparently making a phone call.
The men got up with some difficulty and "started to try and walk off" when the actor came out again and she thought something was said, she told the court.
"The next thing I knew, the neighbour had stepped forward and he had slapped one of the gentlemen, the one without the top on, with his right hand and then punched him on the head with his left hand," she said.
Asked by prosecutor Michael Bunch if there was anything that "precipitated that action" or if the men had been aggressive towards the actor, Mrs Davidson said: "No."
She said she was "100% sure" Mr Tompkinson had moved towards the two men and told the court Mr Poole "stumbled backwards" and fell.
"He just went straight back and his head hit the ground. He just, he didn't move," she said.
"He didn't even put his hands out to stop himself, he just hit the ground."
Crossexamining, Mr Lumley suggested to Mrs Davidson she had seen a "reaching out, a push to the face" not a slap.
She replied: "Absolutely not."
Mr Lumley also said Mr Tompkinson had his phone in his hand throughout the incident after calling the police and "there is no way he could punch with his left hand and slap with his right".
Jurors have heard the actor told police he pushed Mr Poole away in self-defence after the two had come towards him "aggressively".
In a police statement read out in court, Mr Tompkinson referred to Mr Poole, saying: "I wanted to stop him, I didn't want to hurt him."
The trial, which is expected to last another two days, continues.