OMAHA — When Zane Grothe left the pool deck at CHI Health Center Arena late Sunday morning, having failed to make the final in the men’s 400-meter freestyle on the first day of the U.S. Olympic trials, he took with him one precious commodity — a previously secured qualifying time for the Tokyo Olympics in that event — and left behind a messy situation for Team USA.

It would take until Sunday evening, following the final of the 400 free, before that mess could be cleaned up. And even then, after 21-year-old Kieran Smith went wire-to-wire for a dominant, three-second win in the event, the second U.S. spot for Tokyo remained clouded in confusion.

To the relief of USA Swimming officials, Smith’s time of 3:44.86 was comfortably below the 3:46.78 required to qualify for Tokyo — removing the possibility of an unprecedented embarrassment for Team USA, with no one from the final posting an Olympic qualifying time. Runner-up Jake Mitchell’s 3:48.17 was well outside the standard, setting up a complex set of contingencies for the second spot that could take weeks to sort out.

The bottom line: Mitchell, 19, won’t make the Olympic team unless he gets under the qualifying time, either through a time trial or a sanctioned meet by June 27. Mitchell said Sunday night that he intended to do a time trial — swimming alone against the clock — at some point during this meet. If he wants or needs the option of a sanctioned meet, where the presence of other swimmers and a crowd could provide an adrenaline boost, there is one June 26-27 in Mission Viejo, Calif.

If Mitchell fails, one of the swimmers who finished behind him could make the U.S. team in Tokyo instead — provided he hits the qualifying time via one of those same avenues. And if none of them manage it, the spot could revert to Grothe — even though he finished in 11th in the preliminary heats and failed to make the final.

Elsewhere, the opening night of the trials — delayed by a year by the coronavirus pandemic that also pushed the 2020 Tokyo Games to 2021 and contested in front of a raucous crowd that made the noise of a packed house despite being limited to 50 percent capacity — produced the sort of huge swims this meet has become known for.

Michael Andrew set an American record in the 100-meter breaststroke in the morning prelims with a time of 58.19 seconds, then broke it again in the semifinals at night, going 58.14. Assuming Andrew prevails in Monday night’s final to become a first-time Olympian, he would be a legitimate medal threat in an event in which Americans finished sixth and 19th — the latter being Andrew — at the 2019 worlds, the most recent major international competition. He also could fill what had been a gaping hole at breaststroke in Team USA’s 400-meter medley relay.

Chase Kalisz, the silver medalist at the 2016 Rio de Janeiro Olympics in the 400-meter individual medley, won the same event at the trials Sunday night in 4:09.09, earning his spot in Tokyo. As he was about to exit the pool deck, he was engulfed in a bear hug by now-retired, 23-time Olympic gold medalist Michael Phelps, who had bounded down from his seat in the stands to congratulate his former training partner.

On the women’s side, 18-year-old Torri Huske of Arlington broke Dana Vollmer’s nine-year-old American record in the 100-meter butterfly at 55.78 — a time that ranks third in the world all-time and is just three-tenths of a second off Sarah Sjostrom’s 2016 world record of 55.48. Huske, a senior at Yorktown High bound for Stanford, enters Monday night’s final as the top seed, putting her one big swim away from being an Olympian.

Still, in the bigger picture, there were nearly as many questions for Team USA as there were triumphs on Day 1, with the disappointing times behind Smith in the men’s 400 free underscoring the critical lack of depth of the American men and the creeping sense that Team USA’s recent dominance no longer can be taken for granted.

At the Australian Olympic trials in Adelaide, the Aussie women produced a couple of epic swims Sunday that reset the landscape. First, Kaylee McKeown took down Regan Smith’s two-year-old world record in the 100-meter backstroke with a time of 57.45. Then distance freestyler Ariarne Titmus came within half a second of Katie Ledecky’s world record in the 400 free in 3:56.90 — making Ledecky (3:56.46 at the Rio Olympics), for the first time, appear vulnerable in one of her signature events, at least until she swims it here Monday.

The night before, in another eye-opening race, four Australian men went below 3:45 in the 400 free, a time no American has bettered this season (or since 2017, for that matter). The Aussies were led by Elijah Winnington’s 3:42.65, a time no American man has ever equaled.

“The Australians are really lighting it up at trials,” Kieran Smith said about an hour after winning the 400 free at U.S. trials in a time that would have ranked fifth in Adelaide. “… It’ll probably take a couple seconds faster than I was tonight to get on the medal stand.”

While it is still early in the U.S. meet, results from both sides of the world highlighted a point many in the sport saw coming: The Americans’ dominance on the international scene is under unprecedented assault, particularly from Down Under.

At the Rio Games, the U.S. swimmers won 33 medals, 16 of them gold — finishing with more medals than runner-up Australia (10 medals, three golds) by a wide margin. At the 2017 world championships, the U.S. squad did even better, winning 38 medals, 18 of them gold. Australia tied for the second-most medals with 10.

But at the 2019 worlds, Australia closed the gap considerably. The Americans still still had more medals the Aussies, but the margin was just 27-19. In gold medals, it was 14-5. Notably, sprint specialist Caeleb Dressel was the only U.S. male to win an individual gold — though he won four of them, plus two more in relays.

Two years later, with the Tokyo Games approaching at last, the Americans’ lack of depth in the longer freestyle events is becoming obvious. In the case of the 400 free, the U.S. squad has been hurt by the fact that both qualifiers for Rio in 2016, Conor Dwyer and Connor Jaeger, have retired. Neither Dwyer nor Jaeger medaled in Rio, but the former picked up a bronze in the 200 free, while the latter took silver in the 1,500.

The decline of the U.S. men is perhaps best exemplified by the 800-meter freestyle relay, an event the Americans have won at the past four Olympics, with Phelps and Ryan Lochte forming half the relay team in each final. But Phelps retired after Rio, and Lochte, 36, hasn’t been a factor in freestyle in several years and is seeded 53rd of 61 swimmers in Omaha in the 200 free, which is used to select the 800 free relay team.

The U.S. squad doesn’t have a swimmer ranked in the top 10 in the world in the 200 free — after failing to place a swimmer in the final at the 2019 worlds — and could get shut out of the medal stand in the 800 free relay in Tokyo for the first time.

“It’s a cyclical thing,” said three-time Olympic gold medalist Rowdy Gaines, now the chief swimming analyst for NBC. Gaines pointed out that, at the 1988 Seoul Olympics, sprinter Matt Biondi was the only American male to win an individual gold and that a poor performance for the U.S. squad at the 2015 world championships was erased by a solid showing the next year in Rio.

“It sucks that it’s happening this year,” Gaines said, “but I don’t think it’s a statement about the future. I think it’s a statement about the present.”