Phelps, back from a six-month suspension in the wake of his drunk-driving arrest last September, clocked 52.92sec to top the qualifying times in the 100m fly, an event in which he set the world record in 2009 and has won at three Olympics.
"It was just weird racing because it's been so long," said the 29-year-old, who said his plan with coach Bob Bowman for the heat was simple.
"Don't worry about stroke, don't worry about anything, just get in the water and race, and that's what I did," Phelps said.
The 18-time Olympic gold medallist said there were plenty of details to improve on -- perhaps in time for Thursday night's final.
Tom Phillips was second-fastest in the heats with a time of 53.02, and Phelps's longtime rival Ryan Lochte was third-quickest in 53.17.
"Afterwards I can say my stroke didn't feel good, probably needed one less stroke, needed to kick more, my breakout on my start was bad -- I can pick apart the race a hundred different ways," Phelps said. "Just getting in the water and racing is something I was looking forward to.
"I'll try to do something tonight."
The eight months since Phelps last competed, at the Pan Pacific Championships, have been challenging.
His drunk-driving arrest in Baltimore was followed by a stint in rehab, and he was axed from the US team for this year's World Championships in Kazan, Russia.
But Phelps, who confirmed on Wednesday he's committed to a 2016 Rio Olympic bid, says he has emerged with a better understanding of himself as a person and not just a swimmer.
That new perspective, however, has only renewed his passion for his sport.
"I can look in the mirror and be happy, and I think that's something that's challenging for a lot of people in the world," Phelps said.
This week isn't the first time Phelps has hit the comeback trail in the Phoenix suburb of Mesa
He ended a near two-year retirement at the same meeting last year, sparking speculation that he was considering an Olympic comeback in Rio.
Lochte, who has long said he never believed Phelps would stick to the retirement he announced after the 2012 London Olympics, said he was happy to see Phelps back on the blocks.
"My hat goes off to him and I'm just glad he's back," Lochte said. "I love racing him. he's the hardest racer in the world.
"He'll go toe-to-toe with you in any event, and I like that." AFP