Now, he's the first swimmer ever to win the same event at four straight Olympics. Ryan Lochte was left in his wake every time.
In what was billed as the final showdown between two of America's greatest swimmers, Phelps blew away Lochte - and everyone else - to win his fourth gold medal of the Rio Olympics and 22nd overall with a victory in the 200-meter individual medley Thursday night.
Phelps finished a full body-length ahead of the field after dominating the breaststroke and freestyle legs, finishing in 1 minute, 54.66 seconds.
A 12-time Olympic medalist, Lochte had settled for two silvers and a bronze behind Phelps at the last three Olympics in this event.
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This time, he didn't even make the podium. Leading at the midway point, Lochte faded to fifth - nearly 3 seconds behind the winner.
"We bring out the best in each other," he said. "I was a little bummed about my performance, but I was happy for him to get a gold and get one for Team USA."
He's got one more individual event at what he insists will really be his final Olympics - remember, he already retired once - and will be looking to add a fourth straight gold in the 100 butterfly to his staggering resume.
Then, he'll close out these Olympics in the 4x100 medley relay. There seems little doubt he'll go six-for-six.
"I've been able to kind of finish how I wanted to," Phelps said.
It was quite a night for the powerful American team, which picked up two more golds. Ryan Murphy completed a sweep of the men's backstroke events in the 200, but the most socially significant triumph was Simone Manuel tying 16-year-old Canadian Penny Oleksiak for the top spot in the 100 freestyle.
Her fingernails painted red, white and blue, Manuel broke down in tears after her landmark victory in a sport that still struggles to attract people of color.
Campbell and her younger sister, Bronte, were hoping to
battle for gold after teaming up to lead Australia to a victory in the 4x100 free relay.
Neither one made the podium by themselves. Cate was under her own world-record pace at the turn, but had nothing coming back and fell all the way to sixth. Bronte was second at the flip and slid to fourth at the finish.
Murphy extended American dominance of the backstroke events, which goes back to 1992.
The Barcelona Games were the last time the United States lost a men's final in those events. Three days after winning the 100 back, Murphy touched first again in 1:53.62.
Murphy became the third American man in the last five Olympics to take both races. Aaron Peirsol pulled off the sweep at Athens in 2004, while Lenny Krayzelburg claimed both golds at the 2000 Sydney Games.
Australia's Mitch Larkin grabbed the silver in 1:53.96, just ahead of Russia's Evgeny Rylov with the bronze in 1:53.97. The Olympics came to an end for another U.S. backstroke champion.
Missy Franklin finished 14th in the semifinals of the women's 200 back - beating out only two other swimmers. It was a far cry from the London Games, where "Missy The Missile" became only the second American woman to take four gold medals in a single Olympics.
In the only non-American victory of the night, Rie Kaneto of Japan pulled away from Yulia Efimova to take gold in the women's 200 breaststroke.
Kaneto grabbed the lead on the third of four laps and powered to the finish comfortably in front. The winning time was 2:20.30.
Efimova was 1.67 seconds behind, leaving the Russian with another silver medal. She also finished second to American Lilly King in the 100 breaststroke after initially being banned from the Rio Games for her links to the Russian doping scandal. China's Shi Jinglin took the bronze in 2:28.28.