There were fireworks at the weigh-in for tomorrow night's boxing match between Sonny Bill Williams and his opponent, Clarence Tillman III, with both men trying to trade punches.

Tillman had tipped the scales at 128.4 kilograms and cut a very imposing figure while Williams, set to compete in his fifth bout, weighed in at 107.9kg.

After the pair weighed in, they posed for photographers.

The men could be seen whispering in each other's faces before Tillman shoved Williams and landed a glancing right cross.

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Williams responded in kind and the two fighters fell brawling to the floor before managers and trainers jumped in to separate them.

The press conference had been a visibly more tense affair than Williams' other appearances, with Tillman scoffing and shaking his head when the All Black said he would bring the fight to him.

Tillman told reporters he went to the TAB this morning and placed a $1000 bet on himself to knock Williams out.