Usain Bolt sits bang in the middle of a sprawling hotel room on a lazy Sunday afternoon. After he stretches out his hand, smiles and says a soft hello, the world's fastest man becomes a long-legged study in stillness and calm. He barely twitches a muscle while everyone else walks and chatters busily around him. Bolt has settled himself in the centre of a plush sofa as he awaits my questions. His eyes are hidden by dark glasses and he seems very serious, with so little time left before this week's World Championships in Moscow.

Ten days ago, in London, Bolt transfixed a raucous crowd at the Anniversary Games when he won the 100 metres with typical ease, despite a “horrifying” start. His status as the world’s most famous and cherished sportsman remains strong. The track, however, is a much darker place now.

Last month two of Bolt's oldest rivals, his close friend Asafa Powell and Tyson Gay of the US, were shamed within an hour of each other when it was announced they had both tested positive for banned substances.

It was a terrible day for the sport – even if it could be a landmark victory in the slow war against doping. These are troubling times for athletics, especially in Jamaica, where five men and women – including their most successful female sprinter, Veronica Campbell-Brown – have failed drug tests this summer. Bolt’s name has featured in debate surrounding the scandal, which explains why, in London, he spoke more bluntly than usual at a massed press conference: “I know I’m clean.”

This new urgency and clarity of purpose explains Bolt’s personal rigour. As we begin, he chides his friends for chatting in a corner of the room. “Come on,” Bolt says, waving them away, “don’t distract me.”

This is a different Bolt. He is quiet yet forceful, emphatic but reflective too, as he considers what he needs to do to seal his renown as the greatest sporting phenomenon of our time and separate himself still further from the tawdry doubts. Yet his competition at the World Championships will be limited because the list of absent runners is striking. Yohan Blake, his training partner and most talented competitor, who beat him twice last season, will be missing because of injury. No Powell. No Gay. No Blake.

Bolt nods before asserting that he is now driven by a higher motivation. “I know that in Moscow I’ve not only got to win. All these guys are missing, so I’ve got to run real fast to settle any doubts.”

He looks steadily at me when I say the obvious words: "World record fast?"

Young athlete
"Yes, I'd like to do something special in Moscow. Of course it depends on the track, the weather, everything but if I get my technique right, and a good start, anything is possible. That's why I've been working so hard. I want to run fast and show the world again what I can do."

Bolt’s impact runs so deep it seems strange to remember that five years ago, in early August 2008, he was just a promising young athlete with a name that looked like a future headline. Since then he has won six Olympic gold medals with a witty brand of showmanship. He appears more unbeatable than ever – with his world record times of 9.58 in the 100m and 19.19 in the 200m still looking indestructible since he set those blazing marks at the 2009 World Championships in Berlin.

In Bolt’s mind, the best way he can answer the bleak allegations surrounding sprinting is by becoming more dominant and running even faster. He would love to break his 200m record most of all and he wonders if it might even be feasible to shatter the almost mythical 19-second barrier.

His boldness is brave and it underpins my conviction that Bolt is as clean as he insists. Some lesser athletes might feel it would be better just to win rather than fuel more attention with another startling performance. But Bolt does not look much like a man about to wilt beneath the burden of carrying a soiled sport on his back.

“I spoke up because I’ve been doing this since I was 15,” he says. “I’ve broken every record in every one of my events for over 10 years. All I can do is keep working hard and running fast to help the sport.”

Asked how he felt when first hearing Powell had tested positive, he reiterates his shock – “I couldn’t believe what I was hearing”. But, trying to stay loyal to Powell, he adds: “A lot of details still need to be looked at. So, like the rest of Jamaica, we wait to see what happens next.”

Powell is as popular in Jamaica as Bolt; and the ramifications for a sprint-mad island have been serious. “It’s not been good but now it’s down to me to run well and help people focus on that again.”

Does Bolt discuss the prospect of breaking more world records with his coach, the gruff Glen Mills?

“We focus less on times than winning. Since I told him I want to go to Rio [and the 2016 Olympics] and win another three gold medals our target has become very clear.”

Bolt’s extraordinary tilt at sporting history is epitomised by his ambition of winning a third consecutive hat-trick of Olympic races.

“To win three in Rio would mean I am with legends like Muhammad Ali and Pele. So that’s why I’m ready to work so hard and make all the sacrifices of a clean athlete. I can do it.”

A languid smile stretches beneath the shades for, like Ali, he relishes making grand predictions and then bringing them to fruition. This time last year Bolt had just won the 100m at the London Olympics, when he obliterated the field and made warnings that he might lose seem absurd. His own prophecy when we had met a few weeks earlier was that he would beat Blake and everyone else decisively.

Imperious style
Bolt celebrated winning the 100m at London 2012 by posting pictures of himself kidding around in his room with three young women from Sweden's handball team at 3am. It was a familiar Bolt way of preparing himself for the 200m and relay finals – which he duly won in imperious style.

This year has been different. Aware of the need for renewed concentration, Bolt says that, after the Anniversary Games: “I didn’t go out. I’m focused on the worlds. It’s competition time. That’s why the coach is happy.”

When has Mills, a tough trainer slow to offer praise, been happiest with Bolt’s performances? “He’s happiest when I’m training hard. He likes it most, like now, when I’m training in Europe. There are no distractions. He loves that.”

Bolt’s world championship record is not as pristine as his Olympic legacy – when only his difficulties as a teenager at the 2004 Athens Games resulted in disappointment. The worlds have been harder. He might regard two silver medals in Osaka in 2007, behind Gay in the 200m and the relay, as a turning point because it gave him the belief that he could become a champion. But the pain of 2005 and, especially, 2011, lingers.

“Of course I remember,” he says of 2005, in Helsinki, when he reached the 200m final. But he ran badly, injured himself and finished last. “I was in lane one, which is no good when you’re as tall as me.”

His disqualification in Daegu in 2011, after one false start in the 100m final, frames his saddest moment on the track. “Definitely,” Bolt says as we relive the way in which he was reduced to raging against a wall in a tunnel as he realised his chance of another world title had been ruined.

“It was so disappointing because I was in great shape. I felt that the win was mine. It was a hard lesson.”

Bolt and Mills are now aware that his one invincible and invisible opponent is called Father Time. Can he outrun the passing years long enough to sustain his glory in Rio? “It’s going to be tough. Some of these guys, like Yohan Blake, will be in their prime at 26 but we’ll work hard to make sure I can do it.”

He nods at a reminder that he himself will turn 27 this month. “I keep telling my coach I’m old but he says not to think like that. I listen to him.”

Has his body begun to feel the strain? “A little bit but it’s more to do with things like going out. I could go out and train the next day a few years ago. Not now. I need to control my training. I have to put up with ice baths. I take them after training and it’s painful but I do what I’ve got to.”

Bolt has also tempered his interest in dabbling in other sports. His genuine desire to play professional football won’t happen – and there will be less speculation about Twenty20 cricket in Australia. He is hoping to play in the All-Star Champions League football game next year.

Bolt, presumably, will do better than he did as a basketball player in the NBA's All-Star Celebrity exhibition in February when he scored only two points in 29 long minutes on court. A speculative aim to compete in the long jump has also been abandoned: "When I told my coach I wanted to go to Rio we agreed I'd focus only on the 100m and 200m."

Engage happily
He will still engage happily in crowd-pleasing stunts. Beyond his trip on a rocket around the London Olympic stadium last month he was tickled to drive a Formula One car at a Diamond League meeting in Oslo. "I liked that even more than the rocket," Bolt smiles. "That was a lot of fun because I love cars."

Such moments, combined with his comic routines before and after races, mean that the next three years are likely to be an entertaining Bolt extravaganza. There is, however, a new gravitas to Bolt. He has always been professional but he suggests that he is even more ready to deal with increased scrutiny. “

Jesse Owens, the sprinter Bolt most admires, was dogged by injustice and only competed in the 1936 Olympic Games in Berlin, where he trumped the Nazis. If racism was the burden Owens could never escape, Bolt wants to shine on despite the shadows of doping that shroud his sport.

His task, which resumes in Moscow as he enters the 100m heats on Saturday , is to keep running fast and winning clearly and cleanly. It is the best way to restore dented faith in an ancient battle to continually find the world's fastest man. We will eventually discover if he can ever run as fast as he did in 2009, when he was only 22, but it seems more important that Bolt continues to prove the doubters and sceptics wrong. "I'm ready to do it," he says simply, "like always."
Guardian Service