He's all by himself now, and ready to go for more.
The world's greatest swimmer cruised through the anchor leg of the 4x200-meter freestyle relay to earn his record 19th career Olympic medal and 15th gold on Tuesday night, etching a place in history as the most decorated Olympian of all time.
"It has been a pretty amazing career but we still have a couple races to go," he said.
Now his remaining four days in the pool at the London Games are all about putting that mark even further out of reach. Phelps has three events to go — the 200 individual medley, the 100 butterfly and the 4x100 medley relay.
He won't be racing for a medal on Wednesday, when he competes in the preliminaries and semifinals of the 200 IM.
The big race on Day 5 of the swimming competition will be the men's 100 freestyle. James "The Missile" Magnussen of Australia owns the leading time going into the final. Nathan Adrian of the U.S. is next on the list, followed by Cuba's Hanser Garcia.
Other top contenders are world record-holder Cesar Cielo of Brazil and Yannick Agnel of France.
Kosuke Kitajima will try to make some history of his own in the 200 breaststroke. The Japanese star could become the first male swimmer to win the same event at three consecutive Olympics, although he's a long shot after qualifying fifth-fastest for the final. Kitajima's earlier attempt at a threepeat in the 100 breast fell short.
Leading qualifier Kathleen Hersey goes for the first U.S. gold medal in the women's 200 butterfly since 2000, when Misty Hyman stunned Susie O'Neill in Sydney.
Hours after Phelps earned his 18th medal — a silver in the 200 fly — and his 19th with the relay gold, well-wishers ranging from soccer great Pele to Los Angeles Lakers star Pau Gasol to fellow Olympians and teammates past and present tweeted their congratulations.
"To be a part of that is something I'll tell my kids about," relay swimmer Ricky Berens said. "It's just incredible to go down with him."
Phelps tied the record for career medals held by Soviet gymnast Larisa Latynina by finishing second in the 200 fly, getting out-touched by South African Chad le Clos at the wall. Phelps angrily tossed his cap in the water, a scowl on his face.
About an hour later, he returned with his teammates in tow for the relay. Ryan Lochte led off, followed by Conor Dwyer. Berens gave a lead of nearly 4 seconds to Phelps, who churned through the water and touched in a cumulative time of 6 minutes, 59.70 seconds for his first gold of these games.
When Phelps and the others huddled up before the relay, he told them he wanted "a big lead" going into his anchor leg and they delivered.
"I started smiling with like 20 metres to go, the first time I think I've ever done that in a race," he said. "I knew we had gotten it."
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.
"He's one of the toughest racers and one of the best swimmers ever to live," Lochte said. "It's pretty awesome."
In the stands, Phelps' mother, Debbie, and two sisters cheered his feat. Wearing his 19th medal during the victory stroll, Phelps stood on a plastic chair and reached up to take a small U.S. flag from them. With tears in her eyes, Debbie blew a kiss to her only son.
Phelps' night started out on a sour note in his signature event, one that is especially meaningful to his family since his sisters once swum it, too.
He hadn't lost the 200 fly at the Olympics or world championships since Sydney, when he finished fifth as an unknown 15-year-old.
Phelps, after leading the entire race, tried to glide into the wall instead of taking one more stroke. Le Clos took that extra stroke and beat Phelps by five-hundredths of a second.
"I was on the receiving end of getting touched out," he said. "Chad swam a good race. I've gotten to know him a little over the last year. He's a hard worker, he's a tough competitor and he's a racer."
Le Clos pounded the water when he saw the "1'' beside his name.
"He has always been an inspiration to me and a role model," le Clos said. "I've watched all his races a million times and I've run the commentary over and over. Now, I guess I can watch my race."
Phelps hung on the lane rope and buried his face in his hands, disgusted with himself for having squandered what looked like a sure gold. Le Clos won South Africa's second swimming gold of the games in a time of 1:52.96. Phelps finished in 1:53.01, while Japan's Takeshi Matsuda took the bronze in 1:53.21.
"It's obviously my last one," Phelps said. "I would have liked to win, but 1:53 flat isn't a terrible time. When you look at the picture of it, it's a decent time."
The legacy of his achievements is pretty decent, too. But the kid who always said he simply wanted to be the best Michael Phelps hopes someone else is out there dreaming as big as he did.
"Nothing is untouchable," he said.