At the pre-fight press conference, the pair began mouthing off before Tillman pushed Williams and connected with a right hook to the chin of the All Black.
Tillman was then tackled by a member of Williams' camp and fell to the floor.
Tillman got up and left the gym, while Williams told reporters he was surprised at what had happened "especially from a seasoned campaigner like Clarence".
"That's pretty below the belt, but that's all good," he said.
Williams, 26, said he had arrived at the gym expecting simply to attend a press conference.
"Obviously, there's other things on his mind and he wants to bring the fight right now."
He couldn't repeat what Tillman said to him, but said there was no questioning the American's motivation.
"He's definitely coming to fight and that's what I want - a challenge," he said.
"Hopefully we can put it on for the people."
The weigh-in took place at a Hamilton gym on Tuesday morning, with Williams giving away a large weight advantage after stepping on the scales at 107.9kg, while Tillman measured 128.4kg.
The 34-year-old American is so confident he will overcome his four fights, 26-year-old opponent that he has taken up the TAB's odds of $5.50 for a knockout.
"Trust me on this one," Tillman said.
"I'm going to be putting a thousand dollars on myself to win this fight.
"I'm serious and I'll be putting the bet on to finish the fight by knockout."
- With AAP