So when they were living together, two of nine children crammed into the same house in Brisbane's tough outer suburb of Woodridge, just one sport dominated.

"It was actually cricket,'' says Papalii grinning, "and Alex was always the bowler. Alex has five brothers and all of them were good batsman, so they used to make me and my little brother, John, field. Alex always hogged the bowling. Obviously no one wanted to fight him, so he got to bowl."

Rating Leapai's chances in the ring against Ukraine giant Wladimir Klitschko, Papalii ignores $1.03 odds for the long-serving champ and refers, instead, to every fighter's most basic chance. One punch.

"For any fighter it's 50-50 because one punch can change the momentum of a fight," Papalii says.

More than most, Papalii knows Leapai has already risen from the canvas and punched his way out. From a home of hard knocks, this is Leapai's second chance at life and he is fighting for his family, the father of six wanting to prove he's a better man than his history suggests.

Alex Leapai, now 34 and 13 years Josh Papalii's senior, was a promising footballer too, on scholarship with the North Queensland Cowboys. But in 1999 he was banned from rugby league for five years for an attack on a referee.

Worse was to come in 2005, when Leapai was jailed for six months for assaulting a group of bouncers outside a Brisbane nightclub.

These are not skeletons hidden in the family closet. Leapai has detailed them himself in the lead-up to this world championship bout in Germany on April 26. He has told it all, of his marijuana and alcohol use, fuelling an internal rage.

But Leapai, who found God in prison and boxing when he got out, wants people to know he's changed.

"Not a bright past but it just shows that it doesn't matter what you do in the past if you have a mindset to change for the future, there can be success ahead,'' Papalii said. "Definitely he's an inspiration.

"I guess with the alcohol and always in trouble, he's definitely changed that with his boxing.

"No-one's perfect. No-one goes out to hurt people or do damage to anyone else. Accidents happen and people are not in the right frame of mind at times. Alex has been in the situation before, but this shows if he keeps his nose clean, success is laying ahead.

"I'll definitely give him a call this week and send him our prayers and love."

When Luama Papalii relocated with her two boys from New Zealand to Australia in the mid 1990s, when Josh was still barely school age, she was welcomed into her brother's family home, the Leapais.

Even before Luama's husband, Patrick Papalii, joined them later, it was three adults and nine children under the same roof.

Alex Leapai gave Papalii his first pair of boxing gloves and taught him to fight. In Woodridge, Leapai has said on the record before, you "had to know how to use your hands" and how "to handle yourself".

Papalii presents off the field as a quiet, respectful, humble person. On it, he hits hard.

He really came to notice in the rugby league world when, as a 20-year-old, he targeted and dominated Australian Kangaroos backrower Paul Gallen in the 2012 finals for the Raiders.

Papalii has since graduated to become Gallen's Australian teammate.

Gallen won his first professional boxing bout in February this year. Papalii, who still spars with his brother and trains with his father in the pre-season, would like to enter the ring too.

"It's always been a passion of me and my little brother, we're always going at it. Hopefully soon, I can get something down the road, but the main focus is the Raiders at the moment," Papalii says.

"I was a big fan of Mike Tyson and I watched David Tua coming up, the last Samoan boxer who went for the heavyweight title and lost to Lennox Lewis. To have my 'brother' up there in Alex, it's definitely something I'm looking forward to.

"Alex has been fighting these Australian boxers for years now, he's always been asking to fight the best ... he's definitely worked hard, he's not just an overnight success.

"It's a big fight for the family, Alex's first test."

Asked to sum up Leapai, Papalii labels the former delivery driver "a real family man".

"Alex will always be Alex. After this fight he's not going to buy a Hummer or anything like that. He's a second-tier mechanic as well, he fixes cars and does them up. He drives old cars. He's definitely not flash, he'll be wearing jeans and the big baggy shirt he fits out anyway. He's really humble and a real family man."

Papalii wants him to know he has family in his corner too.