Australia's James Magnussen admits he put too much pressure on himself as he suffered a shattering defeat to American Nathan Adrian in the final of the blue riband men's 100 metres freestyle.
A fast-finishing Adrian clocked 47.52 seconds to beat out the 21-year-old Magnussen by just a hundredth of a second.
Canada's Brent Hayden claimed bronze in 47.80 ahead of the Olympic 200m freestyle champion Yannick Agnel of France.
Brazil's world record holder Cesar Cielo finished sixth.
Magnussen, the reigning world champion, was bidding to become the first Australian male since Michael Wenden at Mexico 1968 to win Olympic gold in the event.
"That hurts. I did my best tonight and it wasn't quite good enough," Magnussen said.
"To lose by that amount stings, but I felt a lot of great support from everyone back in Australia.
"It's been a tough Olympics. They say what doesn't kill you makes you stronger so hopefully I come out of this a better swimmer, but most of all a better person.
"I think I got a good touch and maybe thanks to a good touch the closeness of the race was a little bit flattering.
"When you lose by that much you look back and think 'what could I have done better', but I've got no regrets."
Magnussen admitted he had a restless night thinking about his date with destiny.
"It all seems like a blur, there's just so much going on in your mind going into it," he said.
"I don't know if you can notice, but my eyes are pretty bloodshot.
"I didn't have a great deal of sleep but I did my best and it wasn't as quick as trials, but it's a different ballgame here.
"I knew that Nathan Adrian was sticking with me and it was going to come down to that touch, but I thought I got a pretty good touch.
"I had the perfect preparation. I was so confident that I was going to swim fast that I put too much pressure on myself."
Magnussen said the whole Olympics experience had been an eye-opener for him.
"I guess having such a successful young career, I just felt pretty much bulletproof coming into this Olympics," he said.
"It's very humbling and I've got a lot more respect for guys like Michael Phelps."
Australian head swimming coach Leigh Nugent was quick to praise Magnussen.
"I think for James it's been a massive learning experience for him," he said.
"He's going to come out of this the better, no doubt about that."
The 23-year-old Adrian finished sixth behind Magnussen at last year's world championships in Shanghai.
Surprisingly for a nation which has been the dominant power in Olympic swimming, Adrian becomes the first American man to win the event since Matt Biondi at Seoul in 1988.
"I have big hands, I guess," Adrian quipped in reference to the margin of victory.
"I had to swim my race, it's difficult with a guy next to you who is coming home in low 24 (seconds) next to you.
"The second 50 (metres) was nerve-wracking.
"It's pretty amazing seeing a one next to your name.
"I almost started crying in the water.
"This is something that happens every four years.
"It's not who swims the fastest time this year but it's who can get their hands on the wall first here tonight."