Joseph Parker’s big brown eyes light up with childlike intensity when he is asked about Samoa, where his fighting heart was forged and will remain. The WBO heavyweight champion for now is a commuter between his adopted home, New Zealand, his workplace, Las Vegas, and London. Here, he has politely parked his rock-like frame in Anthony Joshua’s manor as he prepares for their unification title fight in Cardiff on Saturday, rugged up against the fading winter chill in fawn designer coat and reluctant to part with the new woollen cap that warms his head.

Fighters are notoriously wary of cold weather that can cause illness as they drive their bodies to the limit. You sense Parker might prefer to be sitting under a palm tree in Upolu, the most populated of Samoa’s 11 main islands, where his mother, Sala, is from and where Robert Louis Stevenson is buried. But his calling is strong. As the 26-year-old says: “I enjoy the pain.”

His parents moved to New Zealand in the 80s, “for a better future”‚ Parker says. “I was born in New Zealand, with my [younger] brother John and [older] sister Elizabeth. Living in New Zealand and being a Samoan, I followed my dad – who is named after Jack Dempsey, although he never boxed, because as a baby someone stood on his leg and caused it to be skinnier than the other one.

“I’m not sure why my grandma called dad Dempsey. There were no TVs back in the day; she probably heard it on the radio. But he loved the sport, and introduced us to it. We developed a passion for it, myself and my brother John.” The younger Parker is an undefeated cruiserweight.

“We started training, following David Tua, and what he was doing at the time, getting inspiration from seeing another Samoan Kiwi do well on the big stage.” As Parker’s trainer, Kevin Barry, pointed out, Tua beat four world title-holders but was unable to win a belt, losing to Lennox Lewis in 2000. Parker has that WBO title.

Tua was a more volatile ring personality than Parker, whose menace is silent and grinding. He does a lot of things very well, but none of them spectacularly so. Yet it does not do to prod the quiet beast. “It is a Polynesian thing where you are calm and relaxed in your approach to life. But, when you’re in the ring, you really go for it,” Parker says, before turning to his family and their membership of the Church of Jesus Christ of Latter-day Saints. “For myself, it’s the offering of my parents, how they brought me up, the standards they instilled in me as a boy, being a member [of the church]. All those things play a big part in how I am, how I approach things. It’s a very important part of my life: parents and church – and your surroundings.”

Parker, good enough to have had rugby trials with Auckland, had his lightbulb moment in the boxing ring when he was 16. “I fought a guy who was about 25. I was worried, because he was older and bigger and stronger. But I went in there and bashed him and I thought: ‘Man, this is pretty cool.’”

From there his career went on a steady upward path. “I made the New Zealand team, won medals around the world. I thought: ‘I’m blessed, I’ve got a good talent.’ I had no idea, though, how good I might be. But I loved the sport.

“My parents took out loans to send me around the world, so I had to give it a go. My dad worked at a steel plant, still does; this will be his 30th year there.”

Since winning the vacant WBO title by beating Andy Ruiz Jr two years ago, Parker has moved to Las Vegas for the weather, the excellent sparring and the connections. “My parents arrived in London from Auckland the same day I arrived from Vegas. I haven’t seen them for about nine weeks. They’re proud, they’re happy. And the great thing about boxing is that now I not only can look after myself, I can look after my parents, and my family and friends.”

His cauliflower left ear came not from boxing but packing down in a rugby scrum as a lock. “My dad said: ‘You’ve got to find the sport you want to really pursue, because you can’t do them half‑hearted.’ I picked boxing. It was the tougher option physically, mentally, emotionally. But, if you love it, you don’t see it as tough.

“I enjoy the pain my body goes through. If you’re in a hard moment in a fight, there’s a lot of things go through your mind. But the most important thing to me is family. I have my parents there, my brother is in my corner, all watching. I also think of my daughter back home.” He and his partner have a second child on the way.

Of his 24 bouts since turning professional in 2012 his eighth stands out as the toughest. Few fans would be aware of what Parker endured that night in Manuka City in April 2014, against Marcelo Luiz Nascimento in a Pacific heavyweight title bout. “I knocked him down in the first round but he got up and busted my ear-drum. It made me lose my balance so I had to really dig deep to come back and beat him.”

Within two years of that seventh-round stoppage he was back in the same ring against Carlos Takam – who would go on to give Joshua such a hard time in the champion’s last defence in Cardiff – and Parker testifies that the Frenchman is as hard as he looks – and as strong as Joshua has claimed.

Like Joshua, Parker has learned to cope with celebrity although it has its own New Zealand component. “There’s a lot of distractions, you get invited to events and people want photos. It’s about the same sort of pressure the All Blacks would have, on that level. When they lose, it’s hell for them. The whole country is on their case.”

He is aware, also, of his country’s boxing tradition, from Kahu Mahanga – who knocked out Anthony Mundine’s father, Tony, in a trans-Tasman showdown that gripped Australia and New Zealand in 1969 – through to Tua and Samoa’s WBA middleweight champion, Maselino Masoe.

When the All Blacks take to a rugby pitch embroidered with the haka, opponents tremble. Parker says Samoa has its own version and there is a line he draws strength from. It translates, simply, as, have a big heart, be strong.

I go back there two or three times a year. It’s a beautiful place. It’s paradise. My parents built three houses there, so we go there for relaxation, to enjoy downtime together. I tell you, the mangoes are beautiful. The whole country will come to a standstill on Saturday. After the fight, I will go back to Samoa. Hopefully, they’ll put on a big parade and we’ll celebrate a great win.”