RugbyL: Double World Cup glory key to Sonny Bill's latest reincarnation

Love him or hate him, Sonny Bill Williams adds a good dose of spice whether it be on the paddock in either code of rugby or in the boxing ring.

The latest reincarnation for the 28-year-old code-swapping Kiwi, who is also New Zealand's heavyweight boxing champion, is back in rugby league, with his aim set on becoming the first player to win both union and league World Cups.

Fresh from helping the Sydney Roosters win the NRL, second-rower Williams is in the New Zealand squad seeking to defend the Rugby League World Cup crown they snatched off Australia in 2008.

Williams, playing as a centre, had also been part of the All Blacks squad that won the Rugby Union World Cup on home soil in 2011 after an acrimonious defection from league (and the Cantebury Bulldogs) in 2008 and going on to play union in France, Japan and back in New Zealand.

"This year, we won a competition (with the Sydney Roosters) and hopefully we can add a World Cup," was Williams' blunt assessment of what he was eyeing here ahead of his team's opening match against Samoa in Warrington on Sunday.

But even his inclusion was masked in controversy after he initially rebuffed a place, only to rescind, meaning coach Stephen Kearney was forced to drop Tohu Harris to make way for Williams.

The second-rower, who dashed the hopes of Waikato Chiefs and the New Zealand Rugby Union by signing a new contract with the Roosters for 2014, added that he thrived on the pressure surrounding him.

"From the start of this season, there was a lot of pressure. I had everything to lose and the pressure was firmly on me," he said.

"I guess I kind of thrive on that. It gets the best out of me. I seem to be able to push that to one side and focus on all the little things.

"It starts with earning my team-mates' respect through doing the hard work that you don't see on TV, not the flashy stuff. I know once I do that, all the big plays, all the flashy things seem to come off."

Williams said that the 13-a-side game was harder for him than union.

"Competitively, I believe rugby league is a lot tougher. Just because in rugby I played in the backs, with the pretty boys kicking stones out wide, doing our hair. In rugby league, I'm in the middle, doing the hard yards. It's a very tough sport."

Williams, who stressed that his boxing career was on hold and that he had no plans for signing for English Super League club Salford, insisted he was now "content".

"I don't know if it shows, but I definitely walk around with a bit of a swagger because I'm happy as a man," he said.

Kiwi captain Simon Mannering was in no doubt about Williams' worth: "He's a huge asset for the side.

"He's great to have in the team, not just for the way he plays but the way he carries himself off the field as well."

Australian skipper Cameron Smith even expressed his hope that the hot-favourite Kangaroos might escape some of the spotlight given the sideshow surrounding Williams.

"That's the type of bloke (Williams) is - a lot of people follow him and they're pretty keen on the way he's playing and what he's doing," Smith told AAP.

"A lot of the interest will be on him, there's no doubt about that, and that's great for us.

"As a lover of rugby league, yes (his inclusion is great), as an Australian representative, no," Smith said.

"He's a fantastic player and he only makes their squad stronger.

"It creates some positive headlines for the sport, but hopefully if there comes a time when we're playing against him, he's not playing that well."

New Zealand coach Kearney added: "When you've excelled at three sports like he has ... it's not only your talent that's getting you through, it's other qualities which have to be a part of his makeup.

"He's as driven as any other athlete I've seen to want to be the best. That helps the group around him."