As so often in a storied career that included 22 Olympic medals -- 18 of them gold -- Phelps made it look easy, powering to a comfortable win in the final heat in 52.84sec to launch a comeback that could take him to the 2016 Rio Olympics.
But Phelps said his first race since the London Olympics brought on a rush of emotions he hadn't felt since his age-group days.
Ryan Lochte, swimming in the heat ahead of Phelps, clocked the morning's second-fastest time of 52.94, which means the longtime friends and rivals -- known for their individual medley battles -- will go head-to-head in Thursday night's final.
"We love racing each other," Phelps said. "Obviously, neither one of us wants each other to win, that's the best thing about it."
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Phelps and Lochte had a friendly exchange behind the blocks -- made possible in part by Phelps's early arrival for his race.
- 'It's amazing' -
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"Fun" is the theme so far of Phelps's comeback, with the 28-year-old saying he's enjoying every relaxed minute.
A cheer went up and cellphone cameras clicked as Phelps mounted the starting block. He delivered his trademark armswing prior to the start and then he was off.
"When you hear the roar of the stands, it's amazing," said Phelps, who just couldn't keep his familiar pre-race scowl in place.