Wellbrock sprinted out to an early lead and was up front most of the way on another sweltering morning in Japan’s capital city.
Even with the race starting at 6:30 a.m. local time, the temperature was already 81 degrees (27.2 Celsius) with 80 percent humidity, making it feel like close to 90 degrees. Unlike the women’s race the previous day, there were no clouds to mitigate the heat.
The stifling conditions apparently got to France’s David Aubry, who dropped out of the race with about 3 kilometers remaining and was carried off the deck on a stretcher. There was no immediate word on his condition.
Britain’s Hector Pardoe also failed to finish.
The 23-year-old Wellbrock pulled away on the final lap to win by a dominating 25.3 seconds, finishing in 1 hour, 48 minutes, 33.7 seconds. It was by far the largest margin of victory in the history of Olympic marathon swimming, which was added to the program at the 2008 Beijing Games.
Wellbrock also won a bronze in the 1,500-meter freestyle on the last day of swimming at the pool.
The silver went to Hungary’s Kristof Rasovsky in 1:48.59.0, while Italy’s Gregorio Paltrinieri picked up the bronze in 1:49.01.1. The Italian was silver medalist in the 800 freestyle at the pool.
Defending Olympic marathon champion Ferry Weertman of the Netherlands finished seventh, while American Jordan Wilimovsky was 10th.
After making the final turn around a red buoy at the far end of the course, Wellbrock blew away the field. His coaches were already cheering and celebrating on the deck long before he entered the finishing lane.
After his dominating victory, Wellbrock climbed from the water and sprawled out on the deck, pouring water on his chest under the blazing sun.
It was a far cry from the 2016 race at Rio de Janeiro, where Weertman beat Greece’s Spyridon Gianniotis by just 0.7 seconds in a photo finish. The top 19 in that event all finished closer to the lead than Rasovsky’s margin behind Wellbrock.
In three previous men’s marathon races, the largest margin of victory was 3.4 seconds by Ous Mellouli at London in 2012.
Mellouli, competing in his fifth Olympics at age 37, finished 20th out of 24 swimmers who completed the race. He was nearly 8 minutes behind Wellbrock.