Sonny Bill Williams has won all four of his professional heavyweight boxing bouts, including two by knockouts.
Yet the dual-code rugby international looks to have an even tougher fight on his hands to reclaim his place in the New Zealand rugby union team for next month's World Cup.
Williams looked a certainty for a starting spot in the tournament opener at Eden Park on September 9 until he was dropped from the side which lost 18-5 to South Africa last weekend.
His lack of versatility, according to Graham Henry, the All Blacks coach, means he cannot even command a place on the reserves bench against Australia tomorrow, New Zealand's final Test before the Rugby World Cup.
Henry's embarrassment of riches at centre - he also has Conrad Smith to select at 13 - will be the envy of Martin Johnson, whose hopes of guiding England to a third successive final appearance look likely to be undermined by a lack of midfield thrust.
Williams's path to the No 12 spot is being blocked by Ma'a Nonu, who is considered the best inside centre in the game.
The fact Williams, who would walk onto any other side in the world, is the All Blacks reserve inside centre illustrates why New Zealand are the overwhelming favourite to lift the Webb Ellis Cup for the first time in since 1987.
"Sonny Bill's challenge is, I think, he's competing against the best No 12 in the world, and that's great," Henry said. "He'll get his opportunity, [but] can he be better than the best number 12 in the world?"
The rugby league convert's first opportunity at the World Cup may come in the All Blacks' second pool game, with New Zealand likely to choose their strongest line-up in the curtain-raiser against Tonga. Williams may have to prove his worth against Japan for a shot at Pool A rivals France on September 24.
"Quite frankly, it's a different experience for Sonny Bill," Henry said."He's always been, and rightly so, the top person in that position in the teams he's played in, whether it be rugby league or rugby union. Now, he's in a situation in the All Blacks where there's high competition for places, and he's not guaranteed a run-on spot. So when he gets the opportunity, he needs to nail it."
But Henry said Williams has responded to his selection disappointment well after being dropped from the side which lost to South Africa last weekend.
"He's been outstanding in the group, and we can't say any more," he said. "Sonny Bill is a very fine rugby player, and a very fine sportsman, and he's desperate for an opportunity. You can't do that by walking down the main street in Brisbane. You have to do that on the football field."