“Unfamiliar and different,” Katie Ledecky called a second-place finish. (Maddie Meyer/Getty Images)

GWANGJU, South Korea — It all felt so foreign, that last lap: the unusual lineup on the medals podium, the shellshocked look on the face of the sport’s most dominant swimmer. These felt like uncharted waters.

“Unfamiliar and different,” as Katie Ledecky later put it.

Ledecky had never lost a 400-meter freestyle race on a major international stage. She was just 16 when she won the 400 at the 2013 FINA world championships, and then she just kept on winning the race handily — twice more at worlds and also at the Olympics. She seemed to be miles ahead of the rest of the world — until Sunday at the world championships, where Australia’s Ariarne Titmus chased her down over the final 50 meters, scoring a staggering upset over the five-time Olympic champion.

Ledecky looked visibly stunned after the race, trying to figure out what went wrong, why her legs couldn’t power her to the wall. She finished in 3:59.97, which was 1.21 seconds slower than Titmus, opening these world championships with a second-place finish in one of her signature events.

“Obviously this stings a little,” Ledecky said.

She still has four more events here — the 200-, 800- and 1,500-meter freestyle races and the 4x200 freestyle relay — to try to reach the top of the podium this week. But with the Tokyo Olympics barely a year away, Sunday’s race wasn’t the way she wanted to start the year’s biggest meet.

“Sometimes things just happen,” said Greg Meehan, her coach. “It’s hard when it’s someone of Katie’s persona. … Things happen, even for the superstars."

Expectations probably couldn’t be higher, but the intrigue surrounding the rest of the meet is now turned up a few notches — not just because Titmus upset the 14-time world champ but because of the dramatic way it all unfolded.

Titmus, 18, led from the start and held a slight advantage heading into the 150-meter turn. But Ledecky caught Titmus and slowly inched ahead, leading the field into the last turn. She was about half a body length ahead of Titmus with 50 meters of water ahead of her, which seemed to be a comfortable margin. After all, had anyone ever seen Ledecky get chased down with that kind of lead?

“No,” said Meehan, who’s also the U.S. women’s team coach. “Of course not.”

At the final turn, Ledecky led by 0.62 seconds, a sizable deficit for the young Australian — one that would have been insurmountable for most any swimmer at most any meet. But Titmus showed she’s not like most swimmers and turned in a monstrous final lap just as Ledecky uncharacteristically slowed down.

“I just got to the last turn and felt like I tightened up,” Ledecky said. “My legs were just dead, and obviously Ariarne took advantage of that and had a heck of a swim.”

Titmus found another gear, and as the crowd began to roar, she started reeling in Ledecky, turning on the jets for the final 25 meters and sprinting past the American star to the wall. Titmus covered the race’s final 50 meters in just 29.51 seconds — her fastest lap — while Ledecky’s final 50 meters (31.34 seconds) was the second slowest of the eight-woman field, a full 1.83 seconds slower than Titmus’s.

The Australian swimmer was well aware of the oddity that had just taken place — “That’s not something that happens every day,” she said — even as others around the pool deck were still processing the ramifications.

“Before the race my coach said to me just fight,” Titmus said. “I never thought I’d find myself in a situation where I’d be mowing Katie down. She’s such a champion.”

Ledecky was trying to become just the third woman to win an individual world title four times in the same event. Her time in Sunday’s final was 3.51 seconds off her world record mark, set at the 2016 Rio Olympics. While she’d hoped to peak here in Gwangju, Sunday’s time was only her third-fastest 400 race this year.

Earlier in the day, Ledecky posted the fastest 400 qualifying time (4:01.84) and easily advanced to the final. She barely kicked at all, hoping to save her legs for the final. “Tonight I felt great. Honestly, my physical preparation has been great for this meet,” she said. “Really expected to be a lot faster than that.”

The 400 has long been a dominant distance for Ledecky. She won the race at the 2014 Pan Pacific championships by more than six seconds, but four years later she saw her closest finish to date, beating Titmus by 1.16 seconds at last summer’s Pan Pacs in Tokyo. The Australian last year became just the third female swimmer to break four minutes and entering the week was viewed as someone who might finally be capable of pushing Ledecky to the wall.

“I’ve only raced somebody that’s gone under four minutes two or three times,” Ledecky said. “It’s an unfamiliar race for me. It’s something I can’t really replicate back home when I swim meets.”

While many look at this year’s world championships as a preview of sorts for next summer’s Olympics, Sunday’s duel offered a peek at how intriguing this Gwangju meet could be. Titmus and Ledecky will square off again in the 200, an event Ledecky hasn’t won in an international competition since her Olympic gold in 2016. Titmus owns the world’s fastest time this year at that distance.

Ledecky returned to the Gwangju pool Monday morning tasked with a new race and a chance for a fresh start. The 1,500 is her most dominant event, and in her preliminary heat, Ledecky posted the day’s top time: 15:48.90. The 1,500 final is Tuesday.

“I need to rebound from this,” Ledecky had said Sunday night, “and I need to get my fight back."

With a busy week still ahead, Meehan said he's certain Ledecky will be able to reset, despite the unfamiliar circumstances and the unexpected 400 finish.

“I’d be disappointed if she wasn’t disappointed,” he said. “She’s earned the right to have any kind of reaction to that that she wants. The expectations every time she races are pretty significant. Just like any of our athletes that have a swim that’s not where they want it to be, there’s going to be that disappointment, sadness, frustration, whatever you want to call it. But I’m confident that she’s going to come back tomorrow and be good.”