Surfing’s final rounds are underway, thanks to a tropical storm’s waves.

ICHINOMIYA, Japan — The fickle waves at the inaugural Olympic surf competition came roaring to shore on Tuesday, pushed by Tropical Storm Nepartak, leading contest organizers to try to jam the quarterfinals, semifinals and medal matches into one busy day.

Two by two, in head-to-head elimination rounds, many of the world’s best surfers paddled into the churning and unpredictable water, just days after they practiced in tiny thigh-high waves.

One by one they emerged, either sent home or pushed onward toward the first-ever Olympic medals in their sport.

The surprisingly bold conditions led to a star-studded semifinals, beginning with the men: Gabriel Medina of Brazil against Kanoa Igarashi of Japan in one showdown, and Italo Ferreira of Brazil against Owen Wright in the other.

The women were to follow: Caroline Marks of the United States against Bianca Buitendag in the first, and four-time world champion Carissa Moore of the United States against Amuro Tsuzuki of Japan in the next.

The winners would meet to determine the winners of gold and silver medals. The losers would compete for bronze.

The decision to go now was made on Monday night, as the storm churned off the east coast of central Japan, sending swells toward Tsurigasaki Surfing Beach. Surfing’s unique schedule, or lack of one, originally had the contest ending no sooner than Wednesday, but it was determined that the wildest waves were coming a day early.

So two of the world’s best surfers, Kolohe Andino of the United States and Kanoa Igarashi, who is Japanese American and competes for Japan, set out in the rain and wind at 7 a.m. for a high-level quarterfinal. The sloppy storm surf made for difficult conditions for the surfers and a bit of a spectacle for those watching. Igarashi advanced, spoiling Andino’s medal hopes.

“It was hard, but that’s what makes surfing fun,” Igarashi said.

If there was any apprehension about the conditions, Gabriel Medina of Brazil, the world’s top men’s contest surfer of the past few years, pronounced the contest fully on in the next heat. He launched a skyward full rotation, landed smoothly and pounded his chest in celebration. Judges were impressed, too, giving the ride a 9.0, the highest score of the contest to that point.

After all the anxiety over surfing’s debut at the Olympics, with questionable waves and a limited field, the competition produced a stellar group of semifinalists for the men. Medina and Igarashi are scheduled to meet in one, and Italo Ferreira of Brazil and Owen Wright of Australia in the other.

The women’s quarterfinals followed the men’s, and Carissa Moore of Hawaii, the four-time world champion, advanced past Silvana Lima of Brazil.

It was more a roiling stew than a series of sets, better to look at than to surf. Clean rides were sporadic, but the whitewater made for spectacular images.

“The waves go really fast, and then they just dump,” Moore said. “It’s kind of tricky to place your maneuvers in this kind of surf.”

Caroline Marks advanced with a victory over Brisa Hennessy of Costa Rica — a possible gold-medal match with Moore was still possible. She rushed out of the water to take measure of the changing conditions that awaited a couple hours later.

Adapting was key; she had warmed up on one board, then competed on another. The tide was pulling out, and the action was shifting north along the quarter-mile beach. There was a lot of paddling, and fatigue could be a factor at the end of a long day.

“I’m having so much fun,” Marks said. “I’m here because I love surfing, and this is so rad.”

The Olympic field began with just 20 men and 20 women, and no more than two of each from any country. Most of the field was set before the pandemic, so five of the top 10 men and four of the top 10 women in the current World Surf League standings did not participate.

Most glaringly, that left out Filipe Toledo of Brazil, who has finished in the top four each of the past four seasons. And Kelly Slater, maybe the most famous surfer of all, could not secure one of the two U.S. spots.

Worries about the surf conditions for the Olympics began years ago. Japan’s Pacific Ocean coast has long stretches of beaches and good surf, but most compared it to the East Coast of the United States, not the turquoise big-barrel spots in places like Hawaii or Australia. It is more Jersey Shore than North Shore.

There was even talk of holding it in a wave pool, which would build certainty into the schedule and into the waves. But organizers were adamant that surfing is more than just riding. It requires studying the ocean, adapting to the ever-changing conditions.

When Olympic surfers got their first look at Tsurigasaki Beach last week, they were met with thigh-high breakers just a few strokes from the beach. It was easy o be skeptical.

But the storm was coming, and it landed hard.