Sonny Bill Williams has returned to the Chiefs in impressive physical shape © Getty Images

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Sonny Bill Williams is "in good nick" as he prepares to return to Super Rugby with the Chiefs, recording the fifth-best time in the pre-season yo-yo test that has replaced the beep test as a measure of a player's anaerobic conditioning.

The All Blacks centre has returned to the Chiefs, with whom he won the 2013 Super Rugby title, after two seasons with Sydney Roosters in Australia's National Rugby League competition, and head coach Dave Rennie describes his high-profile recruit as "phenomenal".

"He's in good nick and it's a really good signal him coming in and being in such good condition this early in the season," Rennie said at the Chiefs' media session on Wednesday, noting that Williams' preparations to get back in the boxing ring, against Chauncy Welliver in the inaugural Footy Show Fight Night in Sydney on January 31, had been "really positive" for the player and the team alike.

Boxing "puts him in the sort of condition he is in now", Rennie said. "It's highly motivating for him and you need that variation."

Williams said the All Blacks and their campaign to retain the Rugby World Cup they won for the second time on home soil in 2011, were at "the back of my mind but the beauty of the Chiefs is that no position is guaranteed".

"I know I have to work my butt off just to get into the starting side," Williams said of the Chiefs.

Williams produced strong performances for the All Blacks on their end-of-year tour, against the United States Eagles and England, after brief appearances for Counties Manukau in the ITM Cup, but he was also poor against Wales in New Zealand's final Test of the season, and he said the first part of 2015 was "about getting the skills back up to speed".

"Coming back to rugby you have to use a lot more mental aptitude," Williams said. "Mixing it up physically won't stress me too much. But there's a lot of hard work to go."

Rennie said: "The key thing is that he plays well for us. A lot of people want to go to a World Cup, but they've got to play well in Super Rugby first."