Michael Phelps practices for the Arena Grand Prix at the Skyline Aquatic Center on Wednesday in Mesa, Arizona. Photo: CFP

Michael Phelps is playing down expectations about his return to competitive swimming but the most decorated Olympian of all time, with a mind-boggling 18 gold medals, is finding it tough to sell himself as an underdog.

On Wednesday, the 28-year-old fronted a packed media conference on the eve of his eagerly-awaited comeback at a US swimming Grand Prix meet in Mesa, Arizona.

He arrived unshaven, a little plumper than normal and with no swimsuit sponsor.

"I haven't signed any contracts, no one's making me do this. I'm doing this because I want to," he said.

"I'm loving just being back in the water and in a group. I'm even more relaxed than I ever was before. I'm smiling a lot, I'm happier, I'm joking a lot.

"I really am the grandfather of your group now. I'm the old man."

But for a man who had spent his life demolishing records and setting the gold standard for Olympic success, it is a routine few are buying.

Phelps did not reach Olympic immortality through a devil-may-care approach to his sport.

Under the surface, he is the fiercest competitor swimming has ever seen. In a sport where medals are decided by tiny fractions, he has an insatiable quest to always get his hand on the wall first.

The journey

Phelps already knew that after winning eight gold medals in Beijing, the only forward for him was down, so he began the process in London, which he vowed would be his swansong.

He cut back to seven events, winning four golds, and plans to trim that back even further in the future, though neither he nor his long-time coach Bob Bowman would elaborate on which events he would stick with.

"I always have goals and things that I want to achieve and I have things that I want to achieve now," he said. "Bob and I can do anything that we put our minds to.

"That's what we've done in the past so I'm looking forward to wherever this road takes me and I guess the journey will start tomorrow."

While Phelps evaded questions about his chances of winning more gold in Rio, he did break away from his mantra that he is just doing it all for fun.

Rio is still more than two years ago but the timing of his return to competition this week is the first real clue that he has his heart set on adding to his golden stockpile in Brazil.

Even below his best, Phelps would be a strong contender to add to his tally just on the strength of the US relay teams. He has swum in each of the three relay events at the past three Olympic Games, collecting seven golds from the team races.

The key lead-up event before Rio is next year's world championships in Russia, where Phelps would be able to measure his progress against his main rivals for Rio and decide his Olympic program.

But the selection process to make the US team for the world championships is more complicated than usual and will start at this year's national titles in August, which will double as the trials for the Pan Pacific championships in Australia.

The performances of swimmers at that meet will then largely determine who gets picked for the world championships.

And despite everything he has achieved, Phelps has yet to qualify for the US nationals because he has not swum in the official qualifying period, which commenced after he retired.

"I obviously knew that if I really wanted to compete at a really high level then I have to do it this summer," he said.

Phelps Photo: CFP

'Having fun'

Rumors of a comeback began to gather steam last year when Phelps re-registered with the United States Anti-Doping Agency, a mandatory requirement before he could race again.

He began training under Bowman at their base in North Baltimore, initially just to get back into shape after piling on almost 40 pounds (18 kilograms), then more seriously in preparation to race again.

"My highest point was probably 225 (pounds). I raced at 187 in London, so the weight came on pretty quick," Phelps said.

"Last week I was about 194. I've lost a lot of weight and get in decent shape. We've been able to get some good work done in the pool."

The history of sport is full of failed comebacks but Phelps, who will be 31 by the time Rio rolls around, said he was unfazed by what people think about his return if he failed to live up to his previous standards.

"I'm doing this for me. If I don't become as successful as you all think I would be or should be, and you think this tarnishes my career, then that's your own opinion," he said.

"I'm doing this because I want to come back and I enjoy being in the pool and I enjoy the sport of swimming. I'm just having fun."