After two days of low-key competition at Auburn University, he unwound on Sunday night. Powerful enough to schmooze with CEOs but too young to experience Happy Hour, Phelps headed to his hotel bar, not to imbibe but to play arcade games.
With his mother as chaperone, Phelps set his jaw, worked a joystick and tried to crush a handful of teammates, one a 13-year-old girl.
"I can change from being a kid," he said, "to being a professional. That's something I've learned to do over the past few years."
Phelps is among a tiny group of teens who became world-class athletes before they had a chance to grow up. His was a case of the stereotypical boy who couldn't draw within the lines finding a niche in a sport defined by its lanes.
The Athens Olympics will begin Friday with 4 billion TV viewers from around the world clicking to its largest sports spectacle. Among more than 10,000 athletes, none figures to loom as large as Phelps. The 19-year-old wants to join a roster of Olympic legends, a roll call that started with Paavo Nurmi, the "Flying Finn" who won five gold medals in distance running in 1924 and that peaked with Mark Spitz.
Any sports fan who isn't buried in fantasy baseball or NFL camps knows that Spitz, like Phelps an American swimmer, won seven gold medals in 1972, the biggest haul in Summer Olympics history. (In 1980, Russian gymnast Alexander Dityatin won a record eight medals, four of them gold.)
In eight days that will span the next two Saturdays, Phelps could race as many as 18 times, in an unprecedented eight events. He will become the first man to swim five individual events at the Olympics, and anticipates a role on all three American relays.
How ridiculous would it be for Phelps to win eight gold medals?
Only 11 nations surpassed that count at the Sydney Olympics four years ago. Even if Phelps wins just five golds, that's one more than Greece claimed four years ago. India, with 1 billion people, did not produce a gold medalist in 2000.
Like seekers of truth scaling a mountain to quiz a wise man, the curious from every continent have turned off the Jones Falls Expressway to ponder Phelps at the North Baltimore Aquatic Club's base of operations, the Meadowbrook Aquatic and Fitness Center in Mount Washington.
The family that shaped him has been documented. Swim technicians have quantified the physique and technique that have been honed by a regimen that seems sadistic to outsiders.
As an omnipresent commercial for a charge card suggests, Phelps has literally logged enough nautical miles to have swum to Athens and back. Every yard notched, every pretty girl or snow sled he has avoided, every choice made by him - more likely, for him - has been designed with one consideration in mind: the Olympics.
It has brought him world records and riches, but at the start, Michael Phelps was dragged onto the road to Athens, kicking and screaming.
When you feel weak, you feel like you wanna just give up.
But you gotta search within you, find that inner strength.
"Till I Collapse," Eminem
Before Twista's "Overnight Celebrity" became a personal favorite, that track from The Eminem Show doubled as the score for the Michael Phelps Show. It played on his Walkman before most of his four world titles, 12 world-record times and 20 national championships.
"It's a part of me," Phelps said.