The American, the most decorated Olympian in history, finished two tenths behind countryman Ryan Lochte in the 100 metres butterfly final, with a time of 52.13sec.

Phelps, swimming at the Arena Grand Prix in Mesa, Arizona, will also contest the 50m freestyle today.

"It's one meet; it's one race," said the 28-year-old.

"It's a long way from whether I decide to continue or not."

Phelps' long-expected announcement last week that he was coming out of retirement ignited the prospect of him adding to his Olympic medal haul at Rio 2016.

He already has 22 medals - of which 18 are gold - to his name, winning eight out of eight gold at Beijing 2008 and four gold at London 2012, after which he retired.

Lochte said Phelps' presence had pushed him into an even quicker swim: "I probably wouldn't have gone that time if Phelps wasn't in that pool with me," said the 29-year-old.

"We push each other all the time.

"With what he's done for the sport of swimming and him leaving kind of broke my heart a little because I love getting on those blocks and racing him.

"Now that he's back I've got a big old smile on my face."

Michael Phelps (left) congratulates compatriot Ryan Lochte after being pipped to the 100m butterfly race win ©Getty Images





Third in the 100m butterfly was Kenya's Jason Dunford who clocked 53.08.

In the other races, it was a good day for the Americans in the freestyle swims.

Katie Ledecky won the women's 400m, Nathan Adrian won the men's 100m, Allison Schmitt won the women's 100m and Michael McBroom won the men's 400m freestyle.