By Evan Pegden

Sonny Bill Williams is back training with the Chiefs in Hamilton.

Williams says the psychological high of winning a big boxing match is just as important to his rugby as the obvious physical benefits leading into the Rugby World Cup.

And that is precisely the reason he believes it is vital he go through with his scheduled January 31 bout against American Chauncy Welliver in Sydney, even though it comes just before the Chiefs' final pre-season outing against the Waratahs in the same city.

"It's just so physically challenging. Every session is a tough one and for myself it refreshes me mentally, keeps me away from everything else – rugby or league – and just puts me in a place where I'm just thinking about the fight.

"Once that's over I'm feeling physically in the best shape possible and mentally obviously pretty good as well."

But Williams said it would not be the same to just do the boxing training to benefit his rugby without the fight at the end of it.

"You have to [have the fight]. I'm only speaking from my experiences, but because it's so foreign, so scary and so daunting working your way up there that once you do it you know nothing on the field will faze you.

"The mental achievement is just so uplifting you just feel that much more confident – not just as a rugby player but as a person."

The beauty of it was that the benefits were twofold – mental and physical.

"The downside is that I might get knocked out but I think that all adds to it and that's why you train as hard as you do.

"I just know I've played my best footy off the back of my boxing campaigns."

Williams said the birth of his daughter had not only not encouraged him to give up boxing but had added an extra motivating factor.

"It is scary but it is the same thing as playing footy, playing cricket, playing anything – you can get injured."

Williams sees Welliver as more of a challenge to him than his last boxing opponent of two years ago – veteran South African Francois Botha – in terms of his durability.

His own advantage would be his fitness and athletic ability but Welliver also had the advantage in terms of experience.

But Willams said his comprehensive preparation this time meant he would be confident of going the entire eight rounds if necessary.

In fact that fitness is already being proved on the rugby training field since he returned to training with the Chiefs on Monday.

Head coach Dave Rennie said Williams had put in a personal best score in the yo-yo test among a myriad of personal bests already set by the squad in testing.

"He's phenomenal," Rennie said of the shape that Williams was in on arrival back at Chiefs HQ in Ruakura.

"He did a 20.1 in the yo-yo the other day which ranked him about fifth in our group but it's his [personal best] and in fact we've had 59 pbs since we came in in December in various strength categories and conditioning.

"Sonny was impressive and it certainly motivated Anton Lienert-Brown, who was running next to him, to keep going and do a 20.3 to stay ahead of him. Tom Marshall was top with a 21.3 and Damian McKenzie did a 21.2.

"[Sonny's] in good nick and it was a really good signal him coming in now and being in that sort of condition on January 5," he said.

Rennie said Williams' boxing had always proved a big positive for his rugby and he could just as easily injure himself playing rugby as boxing.

Williams said he wanted to take his return to rugby one step at a time and firstly establish a starting spot with the Chiefs, but to be part of an All Black team that became the first to defend a Rugby World Cup title was certainly one of his biggest goals for 2015.