TOKYO — Ariarne Titmus was just 15, just beginning to emerge as a freestyle prodigy in Australia, just about to have her mind blown by an American swimmer four years older splashing across her TV screen. It was August 2016, the summer of the Rio de Janeiro Olympics, and Katie Ledecky was at the height of her powers. They swam all the same events, Titmus and Ledecky, but the former had never seen anyone swim them like the latter.

“I remember watching her races,” Titmus said, “and thinking, ‘This chick is nuts.’”

Ledecky has redefined the outer limits of athletic potential during her historic career, breaking convention with her stroke and her training and applying a sprinter’s mentality to her sport’s longest races, so it was inevitable that someday someone would come along — young, hungry and emboldened by her example — to knock her off her throne.

Based on recent performance, betting odds and popular opinion, it’s possible, even probable, that day has arrived.

When they meet in the pool at the Tokyo Olympics for the first time, in the 400-meter freestyle, Titmus will be widely considered the gold medal favorite — a status that reflects not only the Australian’s victory over an illness-compromised Ledecky in the same event at the 2019 world championships but also the more than four-second margin between their winning times at their respective Olympic trials in June.

“I’m definitely still saying I am the hunter,” Titmus, 20, told Australian media during a pre-Olympics Zoom session last week, perhaps less out of humility than a desire to keep the pressure shifted toward Ledecky. The former is an Olympic rookie, the latter a five-time Olympic gold medalist. “She has way more experience than me in this competition, and she has dealt with the pressure before, and I am coming into unknown territory.”

The Australian media, which often refers to Titmus as “the Terminator,” has gone to great lengths to paint them as bitter rivals, despite Titmus’s deep and obvious respect for Ledecky and Ledecky’s practiced nonchalance about any of her fellow competitors.

“When I see her overseas, everything is very civil, very normal. She’s just a person,” Titmus said last week. “It’s not like this massive rivalry that everyone thinks.”

“She’s somebody that I love racing,” Ledecky, 24, said during Team USA’s training camp in Honolulu this month. “I think we bring out the best of each other. I know she’s going to be fast, and I know she thinks the same about me.”

In their most significant head-to-head duel, at the 2019 worlds in Gwangju, South Korea, Ledecky led Titmus by half a body length entering the final turn, only to see the latter track her down over the final 50 meters to win by more than a second — Ledecky’s first loss in one of her core events in seven years. Ledecky was in Lane 4, Titmus in Lane 5, and because Ledecky breathes almost exclusively to her right, she had no choice but to watch helplessly as Titmus zoomed past.

Afterward, as the Aussie media took care to point out, Ledecky leaned on the opposite lane line to gather herself, rather than reach across to congratulate Titmus. Only later, after Ledecky was scratched from the 200 and 1,500 freestyles, was it understood how sick she was: Her illness, described as a stomach virus, landed her in the emergency room for seven hours.

Titmus enters Tokyo ranked No. 1 in the world this year in the 200 and 400 free and second behind Ledecky in the 800 free, giving her three shots at becoming the first Aussie woman to win an individual Olympic gold since 2008. But more than gold medals will be at stake at the Tokyo Aquatics Centre: Titmus’s time in the 400 last month at Australia’s Olympic trials, a blazing 3:56.90, was less than half a second off Ledecky’s world record of 3:56.44, set during from the Rio Olympics. Ledecky swam a 4:01.27 at the U.S. trials in Omaha and hasn’t been under 3:59 since 2018.

Of all the young swimmers who were energized by Ledecky’s historic showing in Rio, Titmus was an unlikely candidate to emerge as her chief rival and potential successor. Born eight days before the Opening Ceremonies of the 2000 Sydney Olympics, she is a native of Launceston, Tasmania, an island state 149 miles south of the Australian mainland.

By the time she was 14, her swimming prowess had outgrown her surroundings, and the family moved to Brisbane to train with an elite team. She was preparing to depart for Maui for the 2016 Junior Pan Pacific Championships when she witnessed Ledecky laying waste to the competition in Rio.

“Without her, I would in no way possible be swimming the way that I am now. She set this benchmark so high that people have been trying to chase,” Titmus told the Olympic Network in 2020. Watching Ledecky’s Rio performances, she remembered thinking, “ ’She’s doing stuff that no one is going to get near.’ And I never, ever thought at that point that at the next Olympics I would be the person who’s in the battle with her.”

In Brisbane, Titmus first encountered Dean Boxall, the charismatic and controversial coach who helped transform her into the Terminator. With his long hair and vein-popping intensity, he is an unmistakable figure at meets, often stalking along the pool deck and waving his arms as Titmus races. Sometimes at practices, he reportedly shouts Ledecky’s name at Titmus as motivation.

But when Boxall speaks of Titmus, it sounds eerily like the way Ledecky’s coaches have spoken of Ledecky.

“When I was first coaching her, I never thought she could be as good as she is now,” Boxall said of Titmus in a Sydney Morning Herald interview in 2019. “But she had this character of being able to sustain work and be so consistent with her approach. She loves training. She loves all of it. A lot of swimmers just love racing. She loves all of it.”

At that 2016 Junior Pan Pacific meet, the same summer as the Rio Olympics, Titmus won the bronze medal in the 400 free in 4:09.81, roughly 13 seconds off Ledecky’s gold medal time in Rio. By the summer of 2017, she was racing Ledecky head-to-head for the first time at the world championships in Budapest, finishing fourth and closing the gap to six seconds. The summer after that, at the 2018 Pan Pacific Championships in Tokyo, she finished second to Ledecky, just 1.16 seconds back.

It would take Titmus another year to end Ledecky’s unbroken reign. They haven’t met in the pool again since then, their rematch delayed a year by the postponement of the Tokyo Games until this summer.

To this point, Ledecky’s Olympic record is unblemished: four individual events, four gold medals. But since achieving her status as the dominant female swimmer in the world, she has never faced a challenger like Titmus.