Singapore - Chad le Clos, one of the few men to have beaten Michael Phelps at the Olympics, has thrown down the challenge to his old rival, urging him to get back into training so they can have a rematch at Rio in 2016.
The South African pulled one of the biggest upsets of the 2012 London Olympics when he got his hands on the wall ahead of Phelps after a dog fight to win the gold medal in the 200 metres butterfly.
When Phelps retired after London, Le Clos easily won the 100-200m butterfly double at last year's world championships, but the prospect of another showdown with the greatest swimmer of all time has got him excited.
"I really, really hope that he and his team decide to swim in Rio, I really believe it will be great," Le Clos told Reuters on Friday.
"It's added motivation for me... with Michael back, it's really sparked my fire, so to speak."
Phelps and Le Clos had another ferocious battle in London, with the American coming out on top in the 100m butterfly, an event he has won at three successive Olympics.
But the 22-year-old Le Clos has been working hard on his speed since then to avenge that loss and believes he can not only win both events next time, but can also break both world records, currently held by Phelps.
"I believe by Rio, I should be in peak condition," said Le Clos, currently in Singapore for the final round of swimming's annual World Cup series.
"I don't think he will be worse in Rio, I think he'll be back where he wants to be.
"He's a champion in all respects but I believe I can beat him again."
Phelps returned to swimming last year and quickly set about making up for lost time, suggesting he was on course for Rio.
He qualified for the U.S. team for next year's world championships in Russia but lost his place and was suspended for six months by USA swimming after being arrested for drunken driving.
He is currently undergoing treatment and is not certain of making it to Rio though few doubt he will be on the blocks in Brazil, looking to add to his record tally of 18 golds.
Cornel Marculescu, the executive director of world governing body FINA, told Reuters that Phelps would be welcomed back to the sport with open arms.
"Michael Phelps is an icon, he's demonstrated everything he can demonstrate, he's the best Olympic athlete of all time," Marculescu said.
"If he comes back and swims, we'll be very happy, we'll welcome him and I'm sure all the swimmers will be very happy to race against him again.
"Any person has an accident but he is Phelps, he's always going to be Phelps, he has the biggest number of achievements.
"I don't think we have to characterise him by one accident."