In what many saw a stunning upset, undefeated Sonny Bill Williams was viciously knocked out by 48-year-old UFC veteran Mark Hunt.

The fight, which took place at the Aware Super Theatre in Sydney, saw Hunt blast out Williams in the fourth round with a barrage of heavy punches.

At the time of the stoppage, Williams was ahead on the scorecards with cards of 30-27, 30-27 and 29-28.

It marked the first defeat for Williams in 10 fights - and likely could spell the end of the 37-year-old's career.

"At this stage I'm not even sure if I'll fight again. That could be it for me, too," Williams said.

"I'm 37, I've always said that if I cop a lot of punishment, that could be it. If this is the end of my sporting career, it’s been a good one ... whatever is written for me is written for me."

The retired rugby star turned pro in 2009, but often took several years off to focus on his rugby career and other ventures.

The biggest win of his career came in 2013, when he picked up a ten round unanimous decision win over former world champion Francois Botha.

After a long layoff, Williams returned to the sport last year, after being inactive since 2015.

He learned in the fourth round of his bout with Hunt, that one punch can change everything in the heavyweight division