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American Michael Phelps finished second in the 100m butterfly final at the Arena Grand Prix in Arizona - his first event since coming out of retirement.

The 18-time Olympic champion, who retired after the London 2012 Games, was beaten by rival Ryan Lochte.

Phelps, 28, came out of retirement following concerns about weight gain.

"It's one meet; it's one race," said Phelps, who will also contest Friday's 50m freestyle. "It's a long way from whether I decide to continue or not."

American Lochte finished in 51.93 seconds - the second-fastest time in the world this year - with Phelps close behind in 52.13 and Kenya's Jason Dunford third in 53.08.

Phelps' time was more than two seconds outside his world record of 49.82, set in 2009, but comfortably under the qualifying standard required for the United States national championships in August - which will act as a selection event for next year's World Championships in Russia.

"I'm really excited about how things went," added Phelps, when speaking to ESPN. "I do know what I need to do if I want to continue and swim faster."

Lochte, a five-time Olympic champion who beat Phelps to win the 400m individual medley at London 2012, said his fast time was a result of his rival's return to the pool.

"I probably wouldn't have gone that time if Phelps wasn't in that pool with me," said Lochte, 29. "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."