Lydia Jacoby of the U.S. upsets Lilly King in the 100-meter breaststroke.

TOKYO — As expected, an American won gold in the women’s 100-meter breaststroke on Tuesday morning. The surprise was that it was Lydia Jacoby.

Jacoby, a 17-year-old Alaskan, overtook Tatjana Schoenmaker of South Africa in the closing meters to send shock waves through the Tokyo Aquatics Center, where Jacoby’s American teammate, Lilly King, had arrived to defend her Olympic title.

Instead, King finished in third to take bronze. Undefeated since December 2015, King had been aiming to become the first two-time Olympic champion in the event.

King, 24, still holds the world record, which she set in 2017 at 1 minute 4.13 seconds. Jacoby finished in 1:04.95, and King came in at 1:05.54.

“I’m surprisingly OK right now,” King said after the race, adding that she was thrilled for Jacoby. “We love to keep that gold in the U.S.A. family, and this kid just had the swim of her life.”

“When I looked up and saw the scoreboard, it was insane,” Jacoby said.

Also in Tuesday’s finals:

In a stunning upset, Ryan Murphy of the United States settled for bronze in the men’s 100-meter backstroke. American men had not lost in the event since 1992, and Murphy was the defending Olympic champion. The Russians Evgeny Rylov and Kliment Kolesnikov finished first and second.

British swimmers dominated the men’s 200-meter freestyle as Tom Dean won gold and Duncan Scott won silver. Kieran Smith of the United States, the bronze medalist in the 400-meter freestyle, got off to a slow start and could not recover, finishing sixth. He had entered the final with the second-fastest time in the semifinals.

The women’s 100-meter backstroke was a scorcher. Kaylee McKeown of Australia won gold in an Olympic record of 57.47 seconds. Kylie Masse of Canada took silver, and Regan Smith of the United States, who had set an Olympic record in the semifinals, won bronze.