Popovici won the men’s 100 freestyle to add to the 200 freestyle he won on Monday. He is just the second man to achieve the double at a worlds, after Jim Montgomery of the United States in 1973.
The 15-year-old McIntosh clocked 2:05.20 – a world junior record – in the 200 butterfly to pip the three Olympic medallists to the title.
McIntosh finished 0.88 seconds ahead of American Hali Flickinger and 1.12 ahead of China’s Zhang Yufei. American Regan Smith was 1.59 behind in fourth.
Popovici, who set a world junior record of 47.13 in qualifying for the 100, had to fight hard to edge France’s Maxime Grousset by 0.06 seconds in the final with a time of 47.58.
Canada’s Joshua Liendo Edwards, who was leading at the halfway stage, took the bronze.
Two-time defending champion Caeleb Dressel didn’t race after withdrawing from the rest of the competition with an unspecified medical condition earlier Wednesday.
The Americans had to wait until the women’s 4×200 freestyle for their 11th gold of the championships as Claire Weinstein, Leah Smith, Ledecky and Bella Sims finished 2.41 seconds ahead of the Australian team and 3.31 ahead of the Canadians.
McIntosh showed no ill effects from the 200 butterfly earlier as she set a junior record of 1:54.79 over her 200 meters. Only Ledecky and Sims were faster on the night.
Kylie Masse claimed Canada’s second gold on the fifth night of racing at the worlds by winning the women’s 50 backstroke in 27.31 – 0.08 ahead of American Katharine Berkoff and 0.09 ahead of French swimmer Analia Pigree.
Masse became the first swimmer from Canada, male or female, to win three golds at a worlds.
The 20-year-old Léon Marchand then continued his remarkable competition by winning the men’s 200 individual medley in 1:55.22, lowering the French record he set in the semifinals.
Marchand finished 0.49 ahead of American Carson Foster, 1.00 ahead of Japan’s Daiya Seto and 1.21 ahead of Chase Kalisz.
It was Marchand’s second gold at this worlds after the men’s 400 medley on Saturday, when he almost took Michael Phelps’ world record, and his third medal after silver in the 200 butterfly on Tuesday.
“I was a little tired this morning,” Marchand told journalists.
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