He grabbed a first aid kit and went to Lui's aid, concentrating on his arm and shoulder where blood was soaking through his shirt.
A third witness, who saw the attack from his vehicle on the street, also described reaching the wounded Lui, who was trying to stand up after being rammed off his bike by a car.
"He got up," he said. "I told him to sit down."
The court has been told that Lui, 63, was alive when the first police officer arrived but was pronounced dead from blood loss after being taken to hospital. He had 13 stab wounds.
Hemi Rapata Meihana Cahill, 30, is on trial after pleading not guilty to being party to Lui's murder, and assault using a motor vehicle as a weapon.
He has pleaded guilty to the aggravated robbery of Lui's Outlaw motorcycle club patch.
The court was told on Monday that two men had followed Lui on his Harley Davidson motorcycle to take his patch as a "trophy".
The first witness said he was standing on the street near his truck when he saw a car revving "like the muffler was loose" coming down the street.
He said it drove directly into the motorbike on which Lui had just arrived at the Outlaws gang pad on Mersey St.
"It [the car] drove straight into the bike," he said. "It did not slow down at all."
Lui was still sitting on the motorcycle and was knocked off it.
The man said he saw someone jump out of the car and go to the motorcyclist. He then started "punching" the man on the ground.
The third witness said earlier he had seen the man, who was wearing a Mongrel Mob patch, striking the motorcyclist in a "back and forth motion". Then he saw the attacker had a knife.
A fourth man said he saw thought two men were giving Lui "a hiding" when he saw them in his rear-view mirror while driving on the street.
He did a U-turn and saw two men laying into the man on the ground. One was wearing a Mongrel Mob patch.
Earlier, the jury was shown images and videos from a network of security cameras that recorded Lui's last ride before he was fatally stabbed.
The images were collected from security cameras on businesses along the route from the Gull Taradale service station – where Lui filled up his motorcycle – to the gang pad.
Some of the videos show Lui's Harley Davidson motorcycle passing the camera, to be followed seconds later by the maroon Holden car driven by Cahill.
The trial jury has been told that Cahill's associate, Belmont Sonny Freedom Eruiti Te Aonui-Tawhai, 23, has separately pleaded guilty to murder, aggravated robbery and assault with a weapon.
The trial before Justice Christine Grice and a jury of seven women and five men has been set down to last two weeks.
