Ian Crocker set a world record and defeated heavy favorite Michael Phelps in the 100m butterfly at the World Swimming Championships on Saturday.
Crocker's time of 50.98 seconds eclipsed the previous world record that fellow American Phelps had set in Friday's semifinals (51.47).
The 18-year-old Phelps was attempting to break his fifth world record at the championships and join Australia's Ian Thorpe as the only man to win three individual gold medals at the same world championships.
Phelps has two individual gold medals and will swim for a third on Sunday in the 400m individual medley, where he holds the world mark.
Crocker, a University of Texas star who had never set a world record before, seemed stunned at his achievement, holding his arms wide apart with his mouth wide open as he stared at the scoreboard after the race.
The 20-year-old Crocker won the silver medal in the 100m butterfly at the 2001 worlds in Fukuoka, Japan, and was a member of the US' gold-medal winning 400m medley relay team at the 2000 Sydney Olympics.
After taking in the results, Crocker and a disappointed-looking Phelps embraced across the lane divider that separated them during the race.
Phelps' time of 51.10 was the second fastest in history, eclipsing his mark set 24 hours earlier.
Andrii Serdinov of the Ukraine took the bronze (51.59).
"I've wanted to go 51 [seconds] for a long time but I guess I've skipped it that one and I'll take the 50," Crocker said. "I don't know what to think right now I'm in shock. I feel like someone's going to wake me up and it's going to be time to swim today."
Phelps's coach Bob Bowman was also stunned.
"What a fantastic race," he said. "That was the best race I've ever seen. I thought that Ian's race was perfect. You can't ask for more. What a swim. I knew he was going to go fast."
Earlier, Dutch triple Olympic gold medalist Inge De Bruijn won her first gold medal of the championships in the 50m butterfly.
De Bruijn completed her single swim down the Palau St. Jordi pool in 25.84, beating silver medalist and eight-time Olympic champion Jenny Thompson of the US (26.00) and bronze medalist Anna-Karin Kammerling of Sweden (26.06).
Kammerling set the world record of 25.57 last year.
In the 50m freestyle final, Russian world record-holder Alexander Popov kept his numerous Spanish fans content by winning his third gold medal of the championships.
Popov, who won his first Olympic gold medal at the 1992 Barcelona Games, finished in 21.92 to beat British silver medalist Mark Foster (22.20) and De Bruijn's countryman Pieter van den Hoogenband (22.29), the bronze medalist.
Popov's record time of 21.64 was set in 2000 in Moscow.
Popov also won the 100m freestyle and was a member of Russia's winning 400m freestyle relay team earlier in the week.
Germany's Hannah Stockbauer won the gold medal in a tight 800 freestyle finish. Stockbauer (8:23.66) beat silver medalist Diana Munz of the US (8:24.19) by less than a length after 16 trips up and down the pool.
Britain's Rebecca Cook took the bronze medal in 8:28.45.
Another Brit, Katy Sexton, won the 200m backstroke gold medal in 2:08.74, ahead of silver medalist Margaret Hoelzer of the US (2:09.24) and bronze medalist Stanislava Komarova of Russia (2:10.17), who was leading at the 100m mark and had set the fastest qualifying time.