Canadian teen Summer McIntosh clocked a meet-record 3min 59.42sec to win the 400m freestyle at the US Open ahead of American distance great Katie Ledecky.
McIntosh, a four-time world champion, handed Ledecky her first defeat in the event in a US pool in 11 years, seven-time Olympic gold medallist Ledecky taking second in 4:02.38.
McIntosh held the 400m free world record for three months last year. But at the World Championships in Fukuoka, Japan, she finished a disappointing fourth as Australian Ariana Titmus regained the world record in winning gold ahead of Ledecky.
"After that race, I learned a lot about how to get back out after it," said McIntosh, who would go on to win 200m butterfly and 400m individual medley gold at Fukuoka.
"I had so many races after that that I was really happy with. No matter how bad one race is, you really get back up and get back into it."
McIntosh said she was "pretty happy" with the result in her first long-course race of a season pointed at the Paris Olympics.
"I was just really excited going into it and really didn't know what to expect ... I'll just kind of learn through this, learn from this, and kind of start to move forward for the rest of the year."
Ledecky, who hadn't lost a 400m free race in home waters since she was third at the 2012 Olympic trials as a 15-year-old, was coming off a victory in the 800m free on Wednesday.
She, too, is gearing up for Paris, with the US Olympic trials to be held in Indianapolis June 15-23.
The two finished comfortably ahead of third-placed Siobhan Haughey of Hong Kong, who clocked 4:06.32.
Kate Douglass pulled off an impressive double with victories in the women's 200m individual medley and the 50m free.
The reigning 200m medley world champion used strong breaststroke and freestyle legs to seize control of a race led by Torri Huske after the butterfly before Regan Smith gained the lead on the backstroke.
Douglass finished in 2:08.46 to finish ahead of Alex Walsh, whose 2:08.96 gave her silver ahead of Huske (2:09.10).
Less than half an hour later, Douglass returned to win the 50m free in 24.38sec, with Abbey Weitzeil and Huske tied for second in 24.41.
"I'm kind of shocked to win that one. I was pretty focused on the IM today," Douglass said.
American Michael Andrew won the men's 50m free in 21.80sec. Josh Liendo was second in 21.90 and Aruba's Mikel Schreuders was third in 21.93.
Seven-time Olympic gold medallist Caeleb Dressel, continuing his return to the sport after a lengthy break, won the "B" final in 21.99sec.
Chase Kalisz, fresh off altitude training in Colorado, won the men's 200m medley in 1:57.43 with Hungary's Hubert Kos second in 1:57.88 and Trenton Julian third in 1:58.46.