All Black Sonny Bill Williams to box for NZ title

Williams defeated 43-year-old Tongan Alipate Liava'a in June

All Black winger Sonny Bill Williams is to fight for New Zealand's heavyweight title in the fifth fight of his part-time boxing career.

The World Cup winner, 26, will take on Richard Tutaki, who has won 19 but lost 21 of his 41 fights, on 8 February.

Williams is allowed to take part in a limited number of fights as part of his New Zealand Rugby Union contract.

"As a boxer I'm evolving. There's a lot of improvement in me," said Williams, who has won all four of his fights.

Williams, who played the final four minutes as New Zealand beat France in the 2011 World Cup final, added: "[Tutaki] is a seasoned campaigner so hopefully I haven't bitten off too much, but I'll be right."

Williams's last two fights have gone the six-round distance as he secured unanimous decisions over Scott Lewis and Alipate Liava'a.

The former rugby league international is unconcerned by his failure to bring those contests to an early end.

"After the last fight I said to my manager I want to try and step up in class," he said.

"I was offered the chance to fight for the belt and it's not every day you get the opportunity."

Williams' latest bout is less than three weeks before he is due to make his debut for Super 15 side Waikato Chiefs.