They are among the Games’ earliest risers and some of its hardiest competitors, waking well before dawn for a race start at 6:30 a.m. that requires diving into a hot, polluted bay that one competitor likened to a "warm puddle.”

For nearly two hours, they knife a ragged line through the murky water and occasionally get hit by fish, until the end, when they thrash furiously to a finish that belies the languid pace of the 10-kilometer swim and often with just seconds separating gold and silver.

Marathon swimming is much different from the pool competitions that get more attention at the Games. And it is not just because of the longer distance. It is always conducted in open water, and around the world, that means low temperatures, high temperatures, flotsam and jetsam, sea creatures, currents and waves.

It is an accepted part of the challenge, and on Thursday, Florian Wellbrock of Germany met it best, winning the men’s race in 1 hour, 48 minutes, 33.7 seconds.

"The temperature today was the biggest competitor,” he said. "I beat it, and I beat everything in this race.”

He defeated Kristof Rasovszky of Hungary, who came in at 1:48:59, and Gregorio Paltrinieri of Italy, who won the bronze in 1:49:01.1.

On Wednesday, in the women’s race, Ana Marcela Cunha of Brazil won in 1:59:30.8, beating Sharon van Rouwendaal of the Netherlands at 1:59:31.7 in a stroke-for-stroke finale, while Kareena Lee of Australia took bronze at 1:59:32.5.

"It was tough conditions at the end,” van Rouwendaal said. "It got warmer and warmer when we went faster and faster.”

Swimmers pass the refueling station during the men's 10-kilometer marathon swimming event on Thursday.  | AFP-JIJI
Swimmers pass the refueling station during the men’s 10-kilometer marathon swimming event on Thursday. | AFP-JIJI

In Tokyo, the heat and pollution posed challenges beyond the norm.

Despite the early-morning start, the air temperature hovered around 27 degrees Celsius at Odaiba Marine Park, and it felt much hotter. The water temperature, 29 degrees, was not far from the cutoff of 31 degrees set by the sport’s governing body for safe swimming, a measure taken especially seriously after the death from heatstroke of Fran Crippen, an American long-distance swimmer, in an open-water race in the United Arab Emirates in 2010.

Swimmers in an event in the bay before the Olympics likened the water to a toilet bowl, but Tokyo officials insisted that a high-tech filtration system would keep the level of dangerous E. coli bacteria low. And they installed a water circulation system that brings cooler water from the bottom to the surface.

Most swimmers Wednesday acknowledged the challenges but shrugged them off as just part of the sport. They are allowed occasional sips of bottled fluids handed to them on long poles by boaters following them, and several said they had made sure to take advantage of those opportunities.

But churning at race pace for nearly two hours still takes a toll.

Ferry Weertman, a Dutch swimmer, trained in Curaçao. Yet the heat was still a factor as he passed a group of swimmers who "got gassed” midway through the race, chasing the leaders.

"Florian had a big gap in the beginning, and I was just a little behind, and I just couldn’t really catch up,” said Weertman, who finished seventh in 1:51:30.8.

Not everyone was impressed with the heat. Rasovszky, the silver medalist, said he had trained in a lake in his native Hungary where the temperature was more than 32 degrees.

"So this,” he said, "was really cool for me.”

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