This time, it's about taking on fewer and shorter races than the 22-time Olympic medallist did in his prime.

At 28, Phelps is far from being too old to dive into the pool. In recent years, swimmers have successfully competed into their 30s and in the case of Dara Torres, who was 41 at her last Olympics in 2008, won medals.

Phelps will compete for the first time since the 2012 London Games at a meeting in Mesa, Arizona, starting on Thursday.

Bob Bowman, the swimmer's long-time coach, said Phelps is entered in three events - the 50- and 100-metre freestyles and the 100 butterfly.

"I think he's just going to test the waters a little bit and see how it goes," Bowman said. "I wouldn't say it's a full-fledged comeback."

Phelps' camp is downplaying his return, which had been rumoured ever since the most decorated Olympian in history returned to training last autumn and re-entered the US drug-testing programme. His six-month waiting period to be eligible for competition ended in March.

"Since 2004, there's been an extraordinary amount of pressure for him to perform a certain way," Torres said. "That's a great move that they're downplaying it a little bit. For him, it's probably just a training meet. He's probably just trying to get his feel back for races."

In Mesa, Phelps will swim 100 free and 100 fly preliminaries on the first day. Then, if he qualifies, he'll decide which race to swim for the evening finals, Bowman said. He'll swim the 50 free on the second day and might swim the 50 fly "just for fun", the coach added.

"I bet you're going to see a little spark in him that you didn't see in 2012," Torres said. "He's going to have a lot of fun with it."

No one is confirming Phelps has his eye on a fifth Olympics in 2016. But to resume the grind of training and drug testing, surely the Rio Games are on his radar.

Bowman said Phelps is "pretty far" from being back in top form. He's been training Monday to Friday with Bowman's team at the North Baltimore Aquatic Club in his hometown.

"He's gotten back into good shape since September," the coach said. "He can give a good effort and certainly not be embarrassed. He's in enough shape to swim competitively."

Phelps is the most decorated athlete in Olympic history. He captured 18 gold medals and 22 medals overall at the past three Summer Games. He broke Mark Spitz's record for a single Olympics by winning eight gold medals at Beijing in 2008.

If he comes back and doesn't dominate, Bowman said it wouldn't tarnish the reputation of Phelps.

"His legacy is sealed," the coach said.

Phelps won't be coming back for the money, having earned tens of millions of dollars in endorsements during his career. However, his endorsement potential certainly goes up if he's in the water. His presence on the pool deck will give a boost to USA Swimming, which can endlessly market him.

"Anytime you can have the most decorated Olympian in history in the pool, it's a fantastic thing for swimming," said Chuck Wielgus, executive director of USA Swimming. "USA Swimming and Michael share the goal of growing the sport, and his return to competition will surely inspire even more kids to give swimming a try."

Phelps' name recognition goes far beyond the pool deck. He's as well-known as athletes from far more prominent sports, such as NBA star LeBron James, an accomplishment for a swimmer whose sport gets most of its attention in an Olympic year.

Olympian Katie Ledecky agreed that Phelps has nothing to lose by diving back in.

"It's just for his own personal kind of thing," she said. "He's already done so much. Whether he adds a couple more gold medals or not, what he's done has been so incredible, whatever he does next should be accepted by all."

Phelps had vowed that he wouldn't swim into his 30s. Since retiring less than two years ago, he has stayed busy with a chain of swimming schools, a foundation focused on water safety and appearances on behalf of his sponsors. He devoted lots of time to golf and participated in a reality show with famed coach Hank Haney. "I think he's just really enjoying it," Bowman said. "He enjoys the training and being physically fit. He just kind of wants to see where he's at. It's more really for fun. It's been nice for me to see him swim just for the joy of it really."

Depending on his early results, Phelps could compete in the US National Championships in August in Irvine, California, where the team for the 2015 world championships will be selected.

"I wouldn't say it's 100 per cent on the radar," Bowman said. "After Mesa, we're going to sit down and talk about it."