He won the 10 kilometres race in a time of 1 hour 48min 33.7sec, a massive 25.3 seconds ahead of silver medallist Kristóf Rasovszky of Hungary, who finished in 1:48:59.0.

Italy’s Gregorio Paltrinieri clawed his way through the field to finish with bronze in a time of 1:49:01.1.

Wellbrock, winner of an Olympic 1,500 metres freestyle bronze, and Paltrinieri, a 800m freestyle silver medallist, both emulated Tunisia’s London 2012 gold medallist Oussama Mellouli by winning medals in both pool and open water at the same Games. 

Mellouli finished 20th here.

The Netherlands Rio 2016 champion Ferry Weertman was also not a factor in the race as he finished seventh. 

"I need five minutes to realise what happened today." Wellbrock said.

"The first seven kilometres was really easy. 

"The water wasn't really warm, so then, I keep up the pace and the last leg was horrible."

The swimmers had to contend with temperatures which reached 29 degrees and 90 percent humidity at race start time.

"The temperature today was the biggest competitor," Wellbrock said.

"I beat it and I beat everything in this race."

Wellbrock had made his intentions clear from the very start of the race and had established an advantage of 10 seconds before the swimmers had completed the first kilometre.

France's Rio 2016 bronze medallist Marc Antoine Olivier was the first to challenge but later faded to sixth as Rasovszky briefly held the lead. 

Wellbrock broke away in the closing stages and extended his lead stroke by stroke.

Rasovszky put water between himself and the fast-finishing Paltrinieri, the Rio 2016 1,500m freestyle champion, but both men admitted they had been shocked by Wellbrook’s fast start.

"I'm really happy," Rasovszky said. 

"I came here to win a medal and it's a silver now.

"Florian was unbeatable today; he was much stronger than us. In the last lap he was much faster than everyone else and I saw in the last 500 metres Gregorio was coming. 

"I tried to push harder but I had no more speed, so I tried to be before him, and make it impossible for him to catch me."

Paltrinieri moved up from 17th to fourth place in the space of one lap but admitted he had the heat hard to handle.

"It was really long, really tough, and really hot," he said.

"I didn't arrive here in the best condition because of an infection that I had the last month, so I knew it was pretty tough for me to compete with these guys.

"It means a lot because this is my first Olympics in open water, and I've been putting myself in a position mentally to try and focus on the open water more and more in these last two, three, four years. 

"That was the goal, to come here and get on the podium"