Michael Phelps was adamant there would be no retirement U-turn this time as he was beaten to gold by a Singaporean who used to idolise him.
Phelps missed out on a 23rd Olympic gold medal by finishing in a three-way tie for second in the 100 metres butterfly, but has one final opportunity in the 4x100m medley relay on Saturday.
Singapore's Joseph Schooling won butterfly gold in 50.39 seconds, eight years after being a starry-eyed schoolboy and posing for a photo with Phelps.
The 31-year-old American, who plans to retire after his fifth Games, finished in 51.14secs, the same time as long-term rival Laszlo Cseh of Hungary and Chad le Clos of South Africa.
Phelps rejected pleas from his rivals and denied team-mate Ryan Lochte's suggestion he would compete in Tokyo on the day 35-year-old American Anthony Ervin won 50m freestyle gold, 16 years after tying for the title in Sydney.
"I will be in Tokyo, but I won't be competing in Tokyo. No more," said Phelps, who had won the 100m butterfly at three prior Games.
"This is it. I'm not doing it. I swore in London I wasn't coming back. (But) this is final.
"I'm in a better state of mind this time than I was four years ago.
"I am not going four more years. And I'm standing by that. I've been able to do everything I've ever put my mind to in 24 years in this sport.
"That's why I came back after '12. I didn't want to have a 'what if?' 20 years later.
"Being able to close the door on this sport how I want to, that's why I'm happy now."
Of his 27 Olympic medals, only five have been off the top step of the podium, with three silvers and two bronze medals.
Le Clos' London 2012 win in the 200m butterfly left silver medallist Phelps with unfinished business and he returned after an 18-month hiatus. He insists the same will not happen again.
"I probably would've been really, really, really upset losing a race - I'm not the super happiest guy in the world, nobody likes to lose - but I'm proud of Joe," Phelps added.
"He obviously swam the best race. I've been able to watch him grow and turn into the swimmer that he is.
"I'm excited to see what more is to come. I'm ready to retire and I'm happy about it.
"I'm ready to spend time with (newborn son) Boomer and (fiance) Nicole and watch the little kid grow."
Schooling first met Phelps at a USA training camp ahead of the Beijing Olympics, where Phelps won eight gold medals.
He posed for a photo with the then 13-year-old, which has since been shared widely on social media.
Schooling and Phelps also drove around a country club in a golf kart in search of monkeys who had a taste for the USA team's nutrition bars.
Schooling said: "When I saw him I was shell-shocked. I couldn't really smile. I just opened my mouth.
"It's pretty crazy what happens in eight years."
Schooling was struggling to comprehend his achievement as the first Olympic gold medal winner in Singapore's history.
"After the race I was like, 'This is out of this world'," Schooling added.
"(Phelps) looked at me and smiled and was like, 'I know'.
"Walking alongside him and celebrating. I'll really cherish that for the rest of my life."
Phelps, Cseh and Le Clos linked hands before mounting the podium to receive their silver medals.
Like Schooling, Le Clos asked Phelps to continue.
Le Clos finished second to Phelps in the 200m butterfly this time and felt he had let down his parents Bert and Geraldine, who have been diagnosed with cancer.
"I make no excuses," he said.
"In many ways I felt I let a lot of people down by not winning.
"(But) I'll be back from that. This is not me done."
The USA did win three gold medals on Friday night, with Maya Dirado winning the 200m backstroke, and Katie Ledecky the 800m freestyle - in a world record of 8:04.79.
And Ervin's victory came in the 50m freestyle, his second gold in the event after a tie with Gary Hall Junior at Sydney 2000.
Ervin retired from swimming in 2003, aged 22, and spent time drinking and taking recreational drugs - as chronicled in his book.
He made his competitive return in 2011 and finished fifth at London 2012 behind Florent Manaudou.
But the Frenchman was second this time, by 0.01 as Ervin won in 21.40.