'Freak of nature' Sonny Bill Williams hangs up his boots

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Wellington (AFP)

Dual-code superstar Sonny Bill Williams announced his retirement from rugby Thursday but the New Zealander isn't giving up on sport, saying he wants to concentrate on his boxing career.

Williams, 35, a two-time Rugby World Cup winner, confirmed he would no longer play rugby league for Australia's National Rugby League outfit the Sydney Roosters.

"A massive thank you to the public and fans for all the support over years. To my many teammates, thanks for the lessons and help along the way," he tweeted.

"It wasn't a perfect journey, but the lessons have helped shape me into the man I'm proud to be off the field today."

Williams won World Cups in 2011 and 2015 with the All Blacks, as well as numerous Bledisloe Cup titles against Australia and Rugby Championships.

He also won NRL premierships in 2004 and 2013, represented New Zealand in rugby sevens at the 2016 Olympics and won all seven of his professional heavyweight boxing bouts.

In rugby union, Williams played in the centres and was renowned for his miracle one-handed offloads and fierce tackling, also earning a reputation as one of the most dedicated trainers in sport.

Off the field, the devout Muslim used his profile to speak up on causes such as refugee welfare and cultural tolerance.

Former All Blacks coach Steve Hansen last year rated Williams the best athlete he had ever worked with.

"The best athlete I've coached from a pure athlete sense, he's a freak of nature," he said at the time.

The All Blacks paid tribute to the 58-Test veteran.

"The offload king. Thank you for all you've given to New Zealand rugby and fans all around the world, @SonnyBWilliams. Enjoy retirement uso (brother)," the team tweeted.

Williams said retiring from team sports would allow longer preparation for his boxing bouts.

"I spoke to my manager, I said 'Bro I'm keen to get back into the ring, I've got a couple of years, I'm 35'," he told Australia's Channel Nine.

"I spoke to the wife first and she had blank eyes but like usual, she supported me."