Ariarn Ditmus defeated Katie Ledecky in the 400-meter freestyle.

In one of the most anticipated scenes of the Tokyo Games, Australia’s Ariarn Ditmus defeated Katie Ledecki of the United States in the 400-meter freestyle, knocking down the dominating champion by two-thirds of a second.

“Surreal,” Ditmus said, still breathing several minutes after the victory. “This is the biggest thing you can do in your sports career.”

The event was attended by world record holder Ledecky A backward thing. Ditmus beat her at the World Championships two years ago when Ledecky was unwell. She was faster than Ledecky in the distance this year.

Ditmus, the only swimmer to beat Ledecky in a long-distance race at a major meet, and his opponent had a goal in the back, saying that at the Australian Olympics, the American champion would not have things “all the way” in Tokyo.

“I fought with her tooth and nail,” Ledecky said, doing what she never did: Explain a loss. “She swam a smart race.”

Titmus backed up the speech in the pool on Monday morning, extending Ledecky to one of his signature events.

Ledecky came out fast, gaining almost full body length in the first half of the race. Then he flipped at the 300-meter mark, realizing that Ditmus had almost even pulled. From the last turn, Ditmus took the lead, and Ledecky had the fight of his Olympic swimming career in his hands. In the final 15 meters she beat as hard as she could, but there was still a little more to Ditmus.

At a distance of more than 200 meters Ledecky has long been considered untouchable.

But swimming is the ultimate sport of an overview: a swimmer sets unparalleled standards, only to find that the set of new competitors fits in much sooner than anyone expected.

The same thing happened to Titmus, a 20-year-old Tasmanian who has been like the power of nature for the past three years, and it was rare to test Ledecky’s rigor as a rival. On Monday, that toughness came to the whole scene as Ledecky put up a fight, but in the final meter Ditmus was strong enough to beat her.

“I wouldn’t be here without her,” Ditmus said of Ledecki. “He set an amazing standard.”

Ledecky also competes in freestyle races in the 200, 800 and 1500 meters in sports, but unlike anything he experienced one day in the office on Monday. Since the initial heat at 1,500 and 200 meters was scheduled for Monday evening, there was little time to worry about second place.

“I was right there,” he said.