With a lot of help from his friends, Phelps took down the last major record that wasn't his alone, swimming the anchor leg for the United States in a gold medal-winning performance of the 4x200-meter freestyle relay Tuesday night.
About an hour earlier, Phelps took one of the most frustrating defeats of his brilliant career, making a shocking blunder at the finish and settling for silver in his signature event, the 200 butterfly.
That tied the record for career medals held by Soviet gymnast Larisa Latynina, but it was hardly a triumphant moment. Phelps slung away his cap in disgust and struggled to force a smile at the medal ceremony.
But any disappointment from that race was gone by the time he dived in the water on the relay, having been staked to a huge lead by teammates Ryan Lochte, Conor Dwyer and Ricky Berens.
Before the race, they all huddled together, fully aware of their moment in history.
"I thanked those guys for helping me get to this moment," Phelps said. "I told those guys I wanted a big lead. I was like, 'You better give me a big lead going into the last lap,' and they gave it to me. I just wanted to hold on. I thanked them for being able to allow me to have this moment."
Berens handed off a lead of nearly 4 seconds to Phelps, who lingered a bit on the blocks, knowing the only way he could blow this one was to get disqualified. Then he set off on what amounted to four victory laps of the pool - down and back, then down and back again, the roar in the Olympic Aquatics Centre getting louder as he approached the finish.
Lochte stood on the deck, waving his arms. Dwyer and Berens pumped their fists. And Phelps touched the wall for his first gold of the London Games with a cumulative time of 6 minutes, 59.70 seconds.
No one else was close. France's Yannick Agnel swam a faster final leg than Phelps, but it wasn't nearly good enough, his country taking silver in 7:02.77. China was far back in third at 7:06.30.
Phelps might have backed into the record a bit by failing to win any of his first three events at these games, but there's no denying his legacy as one of the greatest Olympians ever - if not THE greatest.
Phelps has 15 golds in his career, six more than anyone else, to go along with two silvers and two bronzes. Latynina won nine golds, five silvers and four bronzes from 1956-64.
"You are now a complete legend," the public-address announced bellowed, accompanied by the Foo Fighters' song "Best of You."
Phelps still has three more races to go before he retires, three more chances to establish a mark that will be hard for anyone to touch.
"It has been a pretty amazing career," he said, "but we still have a couple races to go."