Sonny Bill Williams defeated Scott Lewis in his third professional boxing bout; he is now unbeaten in six © Getty Images

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Sonny Bill Williams hopes to hit the ground running with the Chiefs after his seventh professional boxing match, saying the fight preparation will have him "in the best possible condition - not just physically, but mentally too".

Williams has spent the past month since returning from the New Zealand All Blacks' end-of-year tour of the United States and Britain preparing to fight American heavyweight boxer Chauncy Welliver at Allphones Arena in Sydney on January 31, and he is expected to re-join the Chiefs just days later for their final trial of the Super Rugby pre-season. The Chiefs are working with New South Wales Waratahs to arrange a clash between the two most recent Super Rugby champions at Allianz Stadium in Sydney, and Williams is "excited about potentially playing in that game".

Williams said of his boxing preparation in an exclusive interview with News Corp in Australia: "It will be hard work but it will definitely help me with my rugby, too. My fight will come first, though, and then I can focus on that match afterwards. It's a big year next season in a rugby sense and I know putting myself through this almost tortuous [sic] next six weeks will get me in the best possible condition … it will be hard work, but it will definitely help me with my rugby, too."

Sonny Bill Williams made an immediate impact for New Zealand after returning from rugby league © Getty Images

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Welliver, meanwhile, is "1000%" confident he can beat Williams because he has finally been given enough preparation time to make a difference.

"I've taken a few fights this past year with little to no training," Welliver said in an exclusive interview with Fightnews.com. "I will give team SBW that, they had the honour to give me time to get ready. I am going to show up in shape and I got a little bit of a treat for him.

"I've already started training, already started my dieting, I'm tearing the house down. I've been unmotivated boxing lately and have done zero training in the last few years. My weight is dropping as we speak and, though I won't be close to what I was in 2012, I'll be a great weight for this late stage of my life. Beating Sonny Bill Williams gives me something to retire on."

Welliver, 31, has won 55 of 70 bouts since he turned professional after 84 fights as an amateur. Known as 'The Hillyard Hammer', he has also sparred with Wladimir Klitschko, Mike Tyson, David Tua, Shannon Briggs "and countless other champions and Olympians".

Williams is undefeated in six professional bouts since 2010, but he has not fought since beating Francois Botha in controversial fashion for the vacant WBA international heavyweight title in February 2013. The victory was marred by controversy because the fight was shortened, at late notice and unbeknown to most people, to 10 rounds from the scheduled 12. Botha was not informed of the change.

Williams subsequently said that he would not fight again for at least three years, due to a lack of time as he pursued his football career, and he was stripped of the WBA and New Zealand Professional Boxing Association heavyweight titles.