Fortunately it is an overcast day as Michael Phelps, eight times Olympic champion in Beijing, arrives with his security heavies at Hampstead Heath’s men’s bathing pond. Blazing sunshine and he’d have been treated to the sight of naked men lolling on the nude sunbathing lawn.

As it is, he peers through shades at an already grey and overcast day, the water murky beneath him. His expression is blank. Certainly he does not look like a man high on his success. He has picked up a cold in Beijing, is clearly exhausted after the Games and, it emerges later, was photographed leaving a casino in the early hours of this morning. “I’m having fun, that’s the best thing,” he says, flatly.

He is led away to change in a private room filled with antique gym equipment and the pong of sweaty towels, emerging moments later in the famous Speedo LZR Racer suit, the body-compressing outfit worn by all but a handful of swimming gold medal winners. A small crowd of admiring men and women (some female lifeguards have been allowed in for the shoot) eye him up. “It is quite see-through, that suit,” mutters one of the PR flunkies. Apparently, it’s some sort of prototype. Phelps is big – 6ft 4in with an even greater armspan of 6ft 7in. But he doesn’t seem massive because, although he is extremely muscular – there are appreciative murmurs when he strips halfway – he has a very narrow waist and hangs his head. His torso is unusually long and his legs relatively short. His size 14 feet and huge hands are like paddles. When he is not engaged, however, his mouth tends to gape open like a daydreaming teenager, even though he is 23.

He poses dutifully at the end of the diving platform, showing no inclination to enter the water. Below a wizened, octogenarian regular is sculling slowly on his back, like a geriatric turtle. Phelps seems oblivious – he has the look of a man who’d rather be in bed. But this is payback time for his sponsors, one of whom is Speedo. Phelps is swimming’s first real superstar, and the question now is whether he can transcend his sport to enter the league of Tiger Woods and Michael Jordan.

His achievements are immense. The eight gold medals in Beijing eclipsed the tally of his countryman and fellow swimmer Mark Spitz, whose seven golds in 1972 had been the record at a single games. Phelps’s 14 career Olympic golds is far ahead of a clutch of athletes who have nine. In Beijing he set seven world records. In some races, he destroyed the field. But in the 100m butterfly, the margin between a superb athlete and a superstar was as close as it could be. Coming from behind, he won by one-hundredth of a second. A fingernail. Overall, Phelps’s power, porpoise-like technique, and sheer desire set him apart. He is famous for his preternatural calm before races. He roared with delight when his team mates helped him win his eighth gold in the 4x100m medley relay. But he is understated now. “It’s a dream,” he says when he has changed out of the Speedo suit into three-quarter- length shorts and a T-shirt and we are huddled in a small office off the changing rooms. “But afterwards you just let out a big sigh. You know, like [he sighs]. It’s done, it’s over. You wait four years for it to come along, and then it comes so fast, by the end you are like, ‘Gosh, it felt like it took for ever.’ Mentally and physically draining. You know you just want to be able to relax, recover from everything. I had an awesome time,” he adds evenly.

Nor is he exactly overwhelmed by the “greatest Olympian” tag. “It’s cool,” he shrugs.

“I don’t know what else to say. Being in the same sentence with some of the best athletes and the greatest Olympians who ever competed for their countries...”

He declines to be drawn into the debate over whether a swimmer, who can compete in numerous relays and similar events, merits the “greatest” accolade. “My goal was to swim eight events and try to swim them as best I could. Anybody can say whatever they want about it. It’s a sport that I love, a sport that I grew up in and a sport I want to achieve things in.”

Phelps grew up in Baltimore, the son of a state trooper and a school administrator. His parents divorced when he was seven and he has rarely seen his father since. Will he see him again in the future? He shrugs: “Who knows.” But he comes to life when he talks about his mother. “When you see how hard my mother worked day in, day out, how tough her schedule was… She was dedicated to us and her job and she put food on the table, put clothes on our backs and gave us a place to sleep; and everything else we needed, we always had. We tried to mimic what she did.” His older sisters were both swimmers and he followed them into the water, where he found a refuge from his problems on land. He was bullied at school by kids who threw his hat off the school bus and teased him about having big ears. “And one of my English teachers telling my mom at a parent-teacher conference I was never going to be successful was a little motivator, too… When people do stuff like that, it just motivates me even more. I welcome comments like that.”

He suffered from ADHD and was prescribed Ritalin. After two years, however, he took himself off the medication and says it was swimming that enabled him to do so. “It taught me how to focus and work hard to achieve a goal. My goals are always in my head.” Before Beijing, he kept a piece of paper with his Olympic goals by his bed.

His mother now works for an organisation for parents of children with ADHD and Phelps says he “still can’t sit still today. If I’m out of the swimming pool, sometimes I lose track.” Certainly, when we meet, he has a fidgety, restless quality. He believes that kids can manage without drugs if they try. “If you think you can do something, you can do it. If you doubt yourself or are negative, then don’t even think about it any more, because it’s not possible. I thought sometimes I couldn’t do it, and sure enough I wouldn’t be getting any faster.”

This textbook American positive thinking was instilled in him by his coach Bob Bowman. Phelps was 11 when they met, and Bowman told him then he could become an Olympian. The pair have been together ever since. “He’s an interesting guy,” says Phelps. “He’s a perfectionist. Somebody who loves what he does and cares about his athletes.” He is also very tough and uses what Phelps calls “tools for motivation”. Once, when he knew that the young Phelps only had one pair of goggles with him, Bowman deliberately broke them just before a race. Phelps was forced to swim without goggles. This proved a useful lesson. In Beijing, water got into his goggles in the 200m butterfly and he had to swim blind. He still broke the world record and took gold. What does he think about just before a race? “I’m not thinking,” he says simply. “What can I think about? If I have nerves, that’s going to psych me out and I’m not going to be able to perform as well. I black everything else out. Bob taught me that when you walk in the pool, leave all your emotions at the door. The water seems to be the place for me to get away. No matter what I’m going through, if I’m in the water, I’m fine.” Sometimes, if Bowman believes he needs extra motivation, including on a couple of times at the Olympics, he tells him about negative comments from other swimmers just before he gets on the starting blocks. “He tells me and I turn a switch. If I’m upset I take out all my aggression in the water.”