TOKYO—Katie Ledecky on Wednesday won her first gold medal of the Tokyo Olympics in the Olympic debut of her signature event, the 1,500-meter freestyle. In victory, however, it was also clear how much the rest of the world has caught up to her. 

In the event she has dominated by margins that are almost comical—sometimes half of the 50-meter pool—Ledecky had to fend off a charge from 20-year-old teammate Erica Sullivan to win gold in a time that was more than 17 seconds off of her world record from 2018.

And earlier Wednesday, Ledecky finished fifth in a stacked 200-meter freestyle final. It was the first time ever that she failed to collect a medal in an individual event at an international swim meet. 

“I think people maybe feel bad for me that I’m not winning everything,” said a teary Ledecky following the 1,500, “but I want people to be concerned about other things that are going on in the world, people that are truly suffering.”

Ledecky’s morning in Japan was an indicator of how things are going for the Americans through five days of racing at the Tokyo Aquatic Center. The U.S. is still winning plenty of hardware. They just don’t always end up around the necks of the swimmers who have previously carried the team to world records and gold medals.

It’s happening with both the women and the men. And it owes partly to the pandemic and the one-year delay it caused in getting to the Tokyo Olympics. 

The postponement is rewarding younger athletes who got an extra year to emerge in this Olympic cycle. Veteran Olympians, meanwhile, had another year to age and carry the weight of golden expectations. 

On Tuesday, for example, Americans won gold and bronze in the women’s 100 breaststroke, but it was newcomer Lydia Jacoby who topped Lilly King, the world record holder and mainstay of Team USA who has not lost that race since December 2015.

Lydia Jacoby, left, beat Tatjana Schoenmaker and Lilly King in the women’s 100-meter breaststroke.

Lydia Jacoby, left, beat Tatjana Schoenmaker and Lilly King in the women’s 100-meter breaststroke.

Photo: David G. Mcintyre/Zuma Press

On the men’s side, Team USA failed to get on the podium in the 100 breaststroke, 200 freestyle, and 200 butterfly, Michael Phelps’ signature event. For the first time since 1992, “The Star-Spangled Banner” was not played after the men’s 100 backstroke, after defending gold medalist and world record holder Ryan Murphy took bronze behind two younger athletes from the Russian Olympic Committee.

The Americans also took fourth in the men’s 4×200 freestyle relay on Wednesday—the first time Team USA has failed to medal in any relay at the Olympics.

With a few exceptions, the men outperforming expectations here are the rookies. Kieran Smith won a surprise bronze in the 400 free in his first international meet representing the U.S. Bobby Finke, Smith’s teammate at the University of Florida, set a U.S. record in the prelims of the 800 freestyle. Both are 21.

Perhaps one of the best examples of the turnover on Team USA is Jacoby, the 17-year-old from Seward, Alaska, who admitted that she “really didn’t have a real shot of making the team” in 2020 after winning gold on Tuesday morning. The Tokyo Olympics were an event she planned on attending as a spectator with her parents, who captain boats for whale watching tours. 

King, on the other hand, has for years carried the weight of defending her Olympic gold medal and lowering the world record of 1:04.13 she set at 2017 FINA World Championships. When the Games were postponed in the spring of 2020, King scrambled; Jacoby shrugged.

Jacoby was out of the pool for just two months, relocated to Anchorage over the summer to train at the state’s sole 50-meter pool and spent every other week training there for the next 10 months. Her best time before April of this year was 1:07.57. That would not have made the final at Trials, let alone Team USA.

King, meanwhile, was alternating between driving over an hour to Indianapolis to swim in a family’s indoor pool and zipping up a wetsuit for laps in a pond in Bloomington, Ind. Both were acts of desperation to maintain form. 

“Obviously it wasn’t an easy one to have,” said King of the year leading up to the Tokyo Games.

In sports like swimming, which climax every four years at the Olympics, no stretch of training is more intense than the year leading up to the Games. Due to the pandemic, veterans effectively had to keep up that intensity for twice as long, to say nothing of the period last spring when it was impossible to craft long-term training plans.

“We were getting a daily schedule all the way until March [2021]. We were not able to plan more than that,” said Murphy of his pandemic challenges. Emotionally, it was a grind.

For up-and-comers like Jacoby, however, swimming in Tokyo was an unexpected prize, not where an arduous five-year process was supposed to culminate in success. 

Jacoby couldn’t have known how rapidly she would improve—she raced just four times between January 2020 and Trials. The reigning queen of breaststroke and other elder statesmen got blindsided by the swimmers chasing their wakes. 

Jacoby’s winning time of 1:04.95 is about half a second slower than King swam in Rio. That could be a product of the flipped schedule: preliminary heats in Tokyo take place in the evening with finals in the late-morning to accommodate TV audiences in the U.S. Studies have shown that athletic performance tends to peak in the late afternoon between 4 and 6 p.m.

Chase Kalisz, right, andJay Litherland celebrate after finishing first and second in the men’s 400-meter individual medley.

Chase Kalisz, right, andJay Litherland celebrate after finishing first and second in the men’s 400-meter individual medley.

Photo: Kyodonews/Zuma Press

Chase Kalisz said that getting a full night of sleep between rounds was a struggle. His gold-medal-winning time of 4:09.42 in the 400 IM was nearly three seconds slower than what it took to win silver during the late-night finals in Rio.

“I had a feeling a few days earlier that this wasn’t going to be a race for time, it was going to be a battle of who prepared the most through prelims and finals with the order mixed up,” he said.

Write to Laine Higgins at laine.higgins@wsj.com