The Terminator knocked off Katie Ledecky at the Tokyo Olympics.

Australia’s Ariarne Titmus chased down Ledecky to win one of the most anticipated races of the Summer Games, capturing the gold medal with the second-fastest time in history Monday.

Titmus, who trailed by nearly a full body-length at the halfway mark of the eight-lap race, turned on the speed to touch in 3 minutes, 56.69 seconds.

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Ariarne Titmus of Australia, right, leaves the pool after winning the final of the women's 400-meters freestyle as Katie Ledecky of the United States watches at the 2020 Summer Olympics, Monday, July 26, 2021, in Tokyo, Japan. (AP Photo/Martin Meissner)

Ariarne Titmus of Australia, right, leaves the pool after winning the final of the women's 400-meters freestyle as Katie Ledecky of the United States watches at the 2020 Summer Olympics, Monday, July 26, 2021, in Tokyo, Japan. (AP Photo/Martin Meissner)

Ledecky was the defending Olympic champion and world-record holder. She settled for the silver this time in 3:57.36 -- the fourth-fastest time ever recorded.

"I fought tooth and nail," Ledecky said. "She definitely swam a really smart race. She was really controlled up front. I felt pretty smooth and strong going out and flipped at the 300 and it was like, ‘Oh, she’s right there.’"

And then she was gone.

For the first time in her brilliant Olympic career, Ledecky felt the sting of defeat, dished out by a rival from Down Under who made it clear she was not intimidated by the American star.

"It’s surreal," Titmus said. "Crazy when you make this massive plan for something. It’s probably the biggest thing you could pull off in your sporting career, so I’m over the moon."

No one else was even close. The bronze went to China’s Li Bingjie in 4:01.08.

Ledecky's runner-up finish was another disappointment for the Americans after a dynamic start to the swimming competition.

The powerhouse team won six of 12 medals on Sunday but was shut out of the medals in the first two finals Monday. Torri Huske and Michael Andrew just missed out with fourth-place finishes, then it was Ledecky settling for the second spot on the podium — a very unfamiliar place for perhaps the greatest women's freestyle swimmer in history.

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Ledecky lost an individual Olympic final for the first time after winning the 800 free at the 2012 London Games, then capturing three more golds in the 200, 400 and 800 free at Rio de Janeiro five years ago.

"I knew it was going to be a battle to the end," Ledecky said. "I didn’t feel like I died. She just had that faster 50 or 75. Can’t get much better than that."

Yet, it wasn't good enough.

Titmus lived up to her nickname at the end.

"Honestly, at the 200 I was a little bit worried," the Aussie said. "I knew she would be there. No one is going to come to the Olympics and catch a Katie Ledecky unprepared. I guess I just had to trust myself.

"I tried to stay as composed as I could and use the easy speed that I have. And to pull it off in the back end against someone who has an amazing second half of her race, I’m really proud of that. "

Ledecky will get another crack at Titmus in the 200 free, and the American is heavily favored to repeat in the 800 and add another gold in the 1,500 — a new event for the women at these games.

After racing each other right in the middle of the pool, the swimmers clasped hands when it was over.

They climbed out of the pool together, giving each other a hug.

"I just thanked her," Titmus said. "I wouldn’t be here without her. She’s set this standard for middle-distance freestyle. If I didn’t have someone like her to chase I definitely wouldn’t be swimming the way I am."

Perhaps the surest bet at the pool, Britain's Adam Peaty repeated as Olympic champion in the men’s 100 breaststroke.

Peaty was the world-record holder and the first man to break both 58 and 57 seconds in his signature event. He posted the fifth-fastest time in history (57.37) to blow away the field.

Arno Kamminga of the Netherlands claimed the silver in 58.00, while the bronze went to Italy’s Nicolo Martinenghi in 58.33. Andrew was next in 58.84.

Maggie MacNeil captured Canada’s first gold medal at the pool with a victory in the women’s 100 butterfly.

The reigning world champion touched first in 55.59, edging out Zhang Yufei of China (55.64) for the top spot. Emma McKeon of Australia took the bronze in 55.72, beating the 18-year-old Huske by one-hundredth of a second.

Huske went out fast, as is her style, and appeared to be close to the front with about 10 meters to go. But she faded on her final strokes and just missed a spot on the podium.

Defending champion and world-record holder Sarah Sjöström of Sweden was seventh.

Still to come: Caeleb Dressel beginning his quest for six gold medals starting with the men’s 4x100 free relay.

The American team's six Sunday medals was more than they ever won on the first day during Michael Phelps’ stellar career, which encompassed the last five Olympics.

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Phelps retired from competition after the 2016 Rio de Janeiro Games and is doing commentary for American broadcaster NBC at these Olympics.