TOKYO — The 400-meter individual medley is considered the ultimate test of swimming, its international champions earning the unofficial title of the world’s best all-around swimmers. They are the swimming version of the five-tool player — the five tools being butterfly, backstroke, breaststroke, freestyle and pain tolerance. It is a race, lasting four minutes and change, that Chase Kalisz has spent his entire career trying to master.

Eight years into his international career, that pursuit found its ultimate expression Sunday morning at Tokyo Aquatics Centre, where Kalisz achieved the one thing in the sport that had eluded him: an Olympic gold medal. In the final of the 400 IM at the Tokyo Games, he took down a field of eight with a time of 4:09.42, earning Team USA’s first gold in any sport in these one-year-postponed Olympics.

“This is my lifelong dream,” said Kalisz, 27, a native of Bel Air, Md. “I’ve accomplished everything else in the sport — world titles, NCAA titles, an American record — and this was the last thing I wanted to check off.”

After touching the wall, Kalisz lifted himself onto the lane line but either didn’t have the strength or the desire to stay there and soak in the moment. Before he could raise his arms in triumph, he tipped over backward into the water. When he finally came back up, he splashed his hand in the water, then lifted his arms in the air.

He celebrated with U.S. teammate Jay Litherland, also Kalisz’s longtime University of Georgia and Athens Bulldogs Swim Club teammate, who took the silver and who climbed his way over from Lane 7 to Lane 3 to smother Kalisz in a hug.

Kalisz’s triumph highlighted a dazzling first finals session of the Tokyo Olympics meet for Team USA, which amassed six medals — a gold, two silvers and three bronzes — in four events. “A pretty good start,” as Kalisz put it.

Sunday’s race card was bookended by Kalisz’s gold and a gutsy bronze in the women’s 4x100 freestyle relay, with Simone Manuel anchoring the U.S. quartet to a time of 3:32.81.

Manuel, 24, won gold in the 100 free at the 2016 Rio de Janeiro Olympics but failed to make the final in the same event at the U.S. Olympic trials last month. Her addition to the U.S. lineup Sunday — allowable because she was on the U.S. team in the 50 free — was a mild surprise, but it helped the Americans keep alive their streak of medaling in the event in every Olympics they have participated in since 1912.

“Even though the last couple of months haven’t been the greatest for me, I’ve trained really hard the last 4½, five years,” Manuel said. “So eventually that hard work will show up.”

The Australian relay squad took the gold in a world record time of 3:29.69 — the opening shot in what is expected to be a meet-long battle between the Aussies and Americans for worldwide dominance.

If the U.S. squad does nothing else of consequence in this meet — and with Katie Ledecky, Caeleb Dressel, Lilly King and Ryan Murphy yet to make their Tokyo 2020 debuts, there is plenty of reason to think big things are ahead — it has asserted its top standing in the grueling 400 IMs.

With 19-year-old Emma Weyant taking silver in the women’s race, behind Japan’s Yui Ohashi, and Hali Flickinger winning bronze, the Americans earned the maximum four of the available six medals in the 400 IMs. The U.S. women hadn’t won multiple medals in an Olympic 400 IM since Mexico City in 1968, when Claudia Kolb took gold and Lynn Vidali silver.

Kalisz, a onetime protege of Michael Phelps, was the silver medalist in the 400 IM in 2016 in Rio de Janeiro — a result that ended a run of five straight golds for the United States in the event, dating from Tom Dolan in 1996. But a 2018 shoulder injury led to his failing to make the final at the 2019 world championships, and entering this summer’s Olympic trials his health and fitness were major questions — until he won the event at the trials, then blitzed the field Sunday.

“It was pretty devastating in my head,” Kalisz said of finishing second in Rio and snapping the U.S. string of golds. “I let us down... It motivated me very well.”

Phelps, now retired following the most decorated swimming career in history, was in the NBC booth providing color commentary Sunday as Kalisz — whom he has referred to as a little brother — took down the field. At the 2004 Athens Olympics, Phelps won the first of his record 23 gold medals in the 400 IM. Phelps’s only remaining long-course world record is in the 400 IM: his 4:03.84 from the 2008 Beijing Games. But by his mid-20s he had all but stopped swimming a race Kalisz, on Sunday, called a “younger man’s race.”

“I definitely have a lot more pain in that race than I used to,” Kalisz said.

The coronavirus pandemic upended all sports in all corners of the globe, but few sports were altered to the extent of swimming. As pools shut down and gatherings were banned, some of the world’s elite swimmers spent months out of the water. The halt of major competitions meant an unfathomable two-year span without an international meet.

“This has been a year of massive uncertainty, and everyone has overcome their own challenges,” Kalisz said. “As a team, we had problems even getting over here. [But] those adversities along the way bring us together.”

In normal times, the data points amassed over the course of a season — or in the four-year span of the Olympic quad — provide a good idea of what is to come when the world’s best gather. The clock doesn’t lie: You are what your times say you are.

But at the 2020 Tokyo Games, taking place a year late, most of those data points don’t exist. As the meet began, there was a sense that anything could happen — a sense that was confirmed by the results in the first finals session of the meet.

No result was more surprising than Ahmed Hafnaoui, an 18 year-old from Tunisia, winning the men’s 400 free out of Lane 8 in 3:43.36. American Kieran Smith took the bronze in 3:43.94. In Saturday night’s preliminary heats, Hafnaoui, ranked 17th in the world entering the Tokyo Games, had barely qualified for the final, finishing eighth.

The extra year undoubtedly facilitated the rise of a fresh wave of teenagers, many of whom are the new stalwarts of Team USA. Weyant’s strong finish for the silver medal in the women’s 400 IM completed a stunning rise, undoubtedly aided by the one-year delay of the Tokyo Games. Though competition was limited in 2020, Weyant’s best time in the 400 IM ranked her just 10th in the world.

Also on Sunday, in her semifinal of the women’s 100-meter butterfly, Torri Huske, an 18-year-old from Arlington, swam 56.51 to earn a spot in Monday’s final. Huske, a Stanford commit whose Olympic trials time of 55.66 had her ranked No. 1 in the world entering Tokyo, finished third in her heat and will be seeded fifth in the final. Yufei Zhang of China (55.89) locked down the top seed.

At a Games being held without fans, the Team USA swimmers, socially distanced in a section of seats in the lower bowl, did their best to replicate an Olympic atmosphere for their teammates in the water below. As Kalisz swam the 400 IM, they stood as a group and cheered him on. And when he touched first, they made enough noise to make you forget everything was supposed to be different this time.