Tyler Clary won gold in the men's 200m backstroke, as favourite Ryan Lochte had to settle for bronze.
American Clary clocked an Olympic record time of 1:53.41 seconds, with Japan's Ryosuke Irie taking silver.
"Lochte is not super human after all. That will have shaken him. It has shaken Clary, he can't believe it. Clary was behind after 100 but I thought Lochte looked tired on the last turn."
"It was not how I planned the race playing at all, but I stuck to my gameplan and it worked out for me," said Clary.
"You always think of having a result like that, but when it happens it is pretty incredible."
Clary's compatriot Lochte, the world and Olympic champion, was looking to add a third gold to his London 2012 haul.
The 27-year-old had led at every turn, but his time of 1:53.94 was only good enough for third.
Lochte returned to the pool 30 minutes later for the final of men's 200m individual medley, where he was up against Michael Phelps.
Phelps who came out on top
on this occasion, winning gold and his 20th Olympic medal, with Lochte taking silver.