Hamilton - Sonny Bill Williams's boxing career took off as he scored a first round knockout over American Clarence Tillman in Hamilton on Wednesday night.

VIDEO: Sonny Bill Williams in weigh-in brawl

VIDEO: Sonny Bill Williams's first round victory

According to the Stuff.co.nz website, the win left Williams undefeated from five fights and earned him the New Zealand Professional Boxing Association championship belt.

It also dismissed any doubts about the apparent lack of punching power of the All Black.

After the two boxers sounded each other out early, Williams connected with a good left hook and then finished Tillman off with a flurry of blows that had his opponent with his head hanging out of the ring.

Referee Lance Revill stepped in to stop the fight with one second left in the first round.

There was plenty of feeling going into the clash after their much publicised altercation at Tuesday's weigh-in. And that carried on when they got into the ring together, having to be separated as they banged shoulders and exchanged words.

Tillman left the ring immediately after the fight, not bothering to stay around to see Williams presented with his belt.

There was also ugly scenes when Williams's original opponent Richard Tutaki entered the ring to taunt Williams.

Tutaki was removed with All Black and Chiefs team-mate Liam Messam acting as Williams's minder.

The 26-year-old Williams was giving away 6 cm in height, a massive 21kg in weight and tons of experience to the 34-year-old American who had 21 professional fights going into this bout, including 11 wins with six by knockout.

It was a big night in Hamilton with the crowd at the Claudelands Events Centre including All Blacks Tana Umaga, Wayne Smith, Matthew Cooper, Frank Bunce and the bulk of Williams's Chiefs team-mates.

Williams now turns his attention to Super Rugby with the Chiefs, returning to training on Monday and likely to feature in their final warm-up match.