(CNN) -- He was only a kid, a 15-year-old with braces, when he first dove into the Olympic spotlight.
He didn't win any medal at the Sydney Olympics, but he gave fans a tiny glimpse of what was to come when he swam the last 50 meters of the 200-meter butterfly faster than anyone else in the pool. He moved from eighth to fifth and moved from side note to talking point just like that.
His teammate Tom Malchow told reporters at those 2000 Games, "He's a true competitor, and he's going to be one of the greats."
Malchow likely meant swimming great. But did anyone expect the long, lean teenager from Baltimore to grow up to be possibly the greatest Olympian in history?
On Tuesday, Phelps won his 19th Olympic medal, more than many countries have ever won. And with a gold on Thursday he moved his overall total to 20 -- 16 gold, two silver and two bronze medals.
"It's a combination of everything," he told reporters last week on how he became the biggest name in swimming. "You have to have a great work ethic. Have a strong mind, be motivated. There are so many things that come into it.
"I have a decent package so I can't complain."
The United States has been a world power in swimming since the days of Johnny Weissmuller, but Phelps has elevated its status like no other American competitor.
NBC once broadcast the world championships live, a rarity for the sport, because Phelps was so popular. He was the first swimmer to win Sports Illustrated's Sportsman of the Year.
After every Summer Olympics the number of swimmers increases, but when Phelps won eight gold medals in an electrifying eight days in Beijing in 2008, it grew exponentially more than previous post-Olympic years, according to a 2009 article in the Orlando Sentinel.
Fresh off his record-setting Beijing performance, in an offseason where he was more concerned about tricking out his new apartment than training, he told Sports Illustrated he wasn't worried about the future even though he was out of shape and had put on some pounds.
"When I have to turn the switch back on, I know I can. All I have to do is put my mind to something and that's it, it's done," he said. It took a few years, but after some controversy he became motivated again.
He was banned from competition for three months in 2009 after a tabloid newspaper ran a photo of him appearing to inhale from a marijuana pipe. He almost retired, but decided the London Games would be his sendoff from the sport he began as a wispy child in Baltimore.
Phelps, who started swimming to emulate his sisters at age 7, set individual world records 29 times, and still owns records in six events. In the two competitive races he has left he probably won't be besting those times, now that he's almost a swimming old man (and due to slower swimsuits).
He wraps up a career as the most decorated Olympian ever, perhaps the greatest ever. Some commentators will argue the cases of track and field's Carl Lewis, who missed an Olympics due to a U.S. boycott, or distance runner Paavo Nurmi, who won 12 medals. Some might argue for Bjorn Daehlie, the Norwegian cross-country skier. And there's gymnast Larisa Latynina, whose 18 medals are now second all time.
It's a class for which it doesn't take long to call roll.