The most intense month of training in Sonny Bill Williams' career will be the perfect preparation for locking up a rugby union World Cup spot, according to the All Black himself.

Williams has spent the past month working overtime in the boxing ring to be at peak fitness for his upcoming heavyweight fight on January 29 on the Gold Coast, against Australian Scott Lewis.

But far from disrupting preparation for the most important football season of his career, the former NRL star believes boxing will be his secret weapon this year.

Williams was also "hopeful" of overcoming a lower leg injury before the start of the Crusaders' Super rugby season next month.

"It definitely helps (with my rugby) ... For me the proof is in the pudding," Williams told reporters on Monday.

"In the last two years I've gone from a relatively unknown rugby player to playing for the All Blacks, and I put that down hugely to the fact that I am doing boxing on the side.

"It's something that keeps me on my toes and gives me that edge I believe.

"The last four weeks I've gone pretty hard (in training), I've never trained like that in my life."

Williams, who has it written into his contract that he is able to compete in several fights each year, also refused to rule out another fight before the World Cup, which starts in September.

"If I feel that having a fight during the year will sharpen me up, coming in towards the World Cup, I'll do that," he said.

"I've got it in my contract that I'm allowed a couple of fights a year, but first things first is rugby.

"That's the main goal, trying to be the best rugby player I can.

"Obviously there will be my knockers, but for me it's all about that big picture and playing in that World Cup squad.

"That's why I'm doing what I'm doing and working hard in the gym."

Williams also said he had been working closely with the New Zealand rugby union to ensure that his boxing training wouldn't negatively impact his leg problem.

The 25-year-old suffered the injury against Wales in the All Blacks' final game of their northern hemisphere tour at the end of last year and initial fears were that it was a stress fracture in his foot.

"But it's not in my foot, it's in my (tibia) .... and it was early signs of a stress fracture, it wasn't actually a full stress fracture," he said.

"They said it would take about six weeks to heal and I just wasn't allowed to run.

"The first thing I hit up was obviously if I could bounce around the (boxing) ring, and it's obviously a lot softer than the grass and the pavement, so (New Zealand rugby) were all sweet with it.

"I've been in close contact with the New Zealand doctor and it's all good."

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