Tyson Fury v Otto Wallin: The team behind Fury's transformation, in body and mind

Fury works out with his trainer Ben Davison at the MGM Grand Hotel & Casino in Las Vegas, Nevada

"Tyson is an extremist. He doesn't have a good mood. He has an extreme high or an extreme low."

Trainer Ben Davison knows Tyson Fury in a way few people do.

His role in the rebuilding of a former world heavyweight champion once floored by depression and subsequent weight gain has been critical.

Davison's ring work is complemented by the strength and conditioning expertise of Kristian Blacklock who, for the first time in a four-year relationship with Fury, has had a lean fighter on his hands from the start of a training camp.

"This version now, there is no limit to what we can do," Blacklock says.

As Fury gets ready to face Sweden's Otto Wallin in Las Vegas on Saturday night, the two men give BBC Sport an insight into managing his mind and talk us through his punishing regime.

• Fury on finding 'true happiness'

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• Wallin draws on loss of father for inspiration

'He was 60% against Wilder - he's closer to 100% now'

Explosive movements were out of the question when Fury returned to training under Davison late in 2017, two years after his shock world-title success against Wladimir Klitschko. Likewise long runs. Both activities were too risky for a 28st man.

Davison, still only 26, faced criticism for his inexperience. But Fury took a chance on him and now their deep bond is obvious.

"I'm psychologist, trainer, friend, brother, sometimes father-son to him and sometimes he's father-son to me," Davison says.

"He can walk down the stairs and I can tell what mood he is in.

"If he has a deep conversation at night, he will go away and think about it, so the next day we might have to have a good-spirited morning to build him up. When you're talking deep you don't want to stay there with him as he's an extremist and he will go deep, deep, deep into his thoughts.

"He doesn't do things in half measures and I would never want to take that away from him as that is what makes him the fighter he is."

Famously, Fury had lost 10 stone in a year by the time he fought to a draw with Deontay Wilder in December. No-one - from his own father to those in his team - felt he was fully ready for the fight.

Fury's transformation in around 12 months before facing Wilder in December

Blacklock, who began overseeing strength and conditioning before Fury's win over Klitschko in 2015, describes the Wilder bout as a "roll of the dice".

Recent videos of Fury running up hills to the mountain village of Istan in Marbella show his 6ft 9in frame moving gracefully.

"I don't think he has ever been fully loaded because he has always, always had to lose weight and he was way overweight before the Klitschko camp," adds Blacklock.

"This time he can do anything: explosive jumps, athletic movements, no limitations. He can do things I do with a top middleweight.

"He backed himself at 60% against Wilder. He may not be 100% now - I don't think a fighter ever is - but he is getting near that figure."

Davison and Blacklock share WhatsApp messages with Fury's nutritionist, ensuring his daily calorie intake aligns with his workload. Heavy training days see him take on between 4,000 and 5,000 calories.

Fury performs technical boxing work at 10am, Monday to Friday. On three evenings he will take on a tough strength and conditioning workout at night. On the other two evenings and Saturdays his non-boxing workout is a 'blue-zone run', designed to build endurance without pushing his heart rate beyond set limits.

"He is a routine man," says Blacklock. "So training benefits him mentally too.

"Working on the Klitschko fight I asked him 'what are you going to do when you win the world title?' I thought he'd maybe say 'have a big holiday'.

"He turned to me and said 'I will be depressed'. I laughed it off, but he was like 'no, it will be an anti-climax, I will be depressed'. In hindsight, he was suffering then and it got worse."

Prioritising balance over success

If Fury is a happier, more grounded fighter now, then his attitude finds echoes in his support team.

Davison's life has changed drastically since his appointment but, he points out, he still drives the same car, and wears free Tyson Fury-branded clothes and an old pair of trainers.

And, tellingly, no-one in the camp discusses long-term goals, openly at least.

Blacklock works on non-boxing elements such as strength, endurance and core exercises

But if the Fury of 2015 could beat Klitschko and the overweight fighter who returned to the ring could carry the fight to the big-hitting Wilder, the question now is how far can the lean and focused 2019 model go?

Davison and Blacklock seem very aware of prioritising balance in Fury's life over the pressures that come with big talk and extravagant aspirations.

"You can't look too far ahead or you'll slip up. I think that's what Anthony Joshua's team did," says Davison.

"This is the fight game. If you aren't on the ball, you'll get tipped upside down."

Fury has already seen his life turned upside down. Davison, Blacklock and the wider team have somehow guided their complex charge through those dark times and back to the glitz of Las Vegas fight nights.

But it is the psychological side of that challenge that is clearly uppermost in their minds.

"If he were to have all the success in the world and go back to the mentality of a couple of years ago, the success would mean nothing to me," says Davison.