The 18-time Olympic gold medalist will swim the 100-meter butterfly at the Arena Grand Prix on Thursday, his first competition since retiring after the 2012 London Games.
Phelps is the world and American record holder in the event, which includes rival Ryan Lochte and four-time Olympian Roland Schoeman of South Africa. At 28, Phelps isn't even the oldest swimmer in the event. Schoeman is 33 and there is another swimmer who is 34.
Phelps hasn't competed since winning a gold medal in the 400 medley relay in London, where he won his record 22nd Olympic medal. Other than racing, he's not sure what to expect at the Arena Grand Prix.
"Just being able to get back into that mentality of competition, that's one thing I really loved the most about it when I was really competing in 2012 and throughout my career," he said after practice Wednesday.
"That's something I'm looking forward to experiencing."
Phelps said he decided to end retirement because he missed the sport that has been his entire life.
He tried golf and high-stakes poker in a quest to satisfy his competitive drives. He found nothing compared to pulling on a suit and diving in.
"Looking at a black line for hours on end, I don't know what made me do it," he said, "but I'm having fun."
Phelps resumed training last fall at North Baltimore Aquatic Club in his hometown, spurred on by a younger group of swimmers.
"I really am the grandfather of the group, that's the worst part about it," he said.
The intense pressure that accompanied Phelps every time he stepped on the deck during the height of his career has dissipated. He appeared relaxed, smiling and joking with longtime coach and friend Bob Bowman at his side.
"I'm doing this because I want to," he told a gaggle of reporters and TV cameras. "Nobody is forcing me to do this or that."
Bowman and Phelps frequently clashed during his career, with Phelps rebelling against his coach's hardnosed style.
"He's much happier doing the training," Bowman said. "When he first came back he was so out of shape."
"Easy," Phelps scolded playfully. "Sugarcoat it at least."
At his heaviest, Phelps weighed 225 pounds. He competed at 187 in London, and last week was down to 194.
"It took a while to get to a point where OK, he could do this in public," Bowman said.
Phelps isn't worried about marring the legacy he built over four Olympics.
"I'm doing this for me," he said. "If I don't become as successful as you all think I would be or should be and you think it tarnishes my career, then that's your own opinion. I'm doing this because I want to come back and I enjoy being in the pool and I enjoy being in the sport of swimming."
Phelps was noncommittal about whether his comeback would lead to swimming in the 2016 Rio Olympics, although he admitted that if he wants to compete at the highest level, he has to be ready by this summer.
"I am looking forward to wherever this road takes me," he said.