Aaron Peirsol of the United States made up for a disappointing swim earlier in the week at the world championships, breaking his own world record in the 200-metre backstroke by more than a full second Friday in Rome.
Fellow American Ryan Lochte, the defending Olympic and world champion, pushed Peirsol in the first half of the race and led at the midway point.
But Peirsol blew everyone away over the second down-and-back, pulling away to win in one minute, 51.92 seconds. His previous world mark, set three weeks ago at the U.S. nationals, was 1:53.08.
Japan's Ryosuke Irie claimed silver in 1:52.51, also under the old mark. Lochte faded to third at 1:53.82. Peirsol's record was the 31st world mark of the meet.
Peirsol had failed to qualify for the 100-metre backstroke final, an event the two-time reigning Olympic three-time world champion had dominated for years.
In another disappointment, Canada's Brent Hayden failed to qualify for the men's 50-metre freestyle final, finishing 13th overall in a time of 21.73.
Frederick Bousquet of France won in a championship record time of 21.21.
Hayden continues to struggle in Rome. He faltered in his attempt to repeat as 100 freestyle champion on Thursday, placing fourth just behind Bousquet.
Canada's men also failed to advance in the 4x200 freestyle relay on Friday, finishing 11th in the preliminaries.
The Canadian team of Joel Greenshields of Airdrie, Alta., Colin Russell of Oakville, Ont., and Stefan Hirniak and Blake Worsley, both of Victoria, needed a top-eight finish to move on.
Michael Phelps earned his third gold medal of the championships, and the United States set the 35th world record in Rome, in winning the 4x200-metre freestyle relay.
Phelps swam the leadoff leg and was again bested by Germany's Paul Biedermann, who handed off 1.5 seconds ahead of the American star.
But the U.S. had the deeper team. Ricky Berens seized the lead on the second 200, David Walters maintained the margin and Lochte held off Russia's Alexander Sukhorukov on the sprint to the wall, touching in six minutes 58.55 seconds — a hundredth of a second ahead of the U.S. record set in Beijing.
Russia took the silver and Australia claimed the bronze.
Phelps had his world record broken in the 100 butterfly semifinal by Milorad Cavic of Serbia.
Cavic raced to a time of 50.01 seconds to beat Phelps's mark of 50.22, which the American set earlier this month. Phelps also qualified for Saturday's final in 50.48 during the first semifinal heat.