Tunisia's Oussama Mellouli became the first swimmer to win pool and open water medals at a single Olympics as he surged to the 10km title in Hyde Park.
The 28-year-old, who bronze in the
in the pool at London 2012, powered away with 3,000m to go.
Daniel Fogg, who qualified for the race
ahead of Beijing silver medallist David Davies,
put in a late push to claim fifth for Great Britain.
Germany's Thomas Lurz was second, with Canada's Richard Weinberger third.
Fogg told BBC Sport: "I felt quite ropey at one point but on the last lap I had quite a bit left and I was feeling good. I was really happy with my first lap, but the pace slowed down, it bunched together and I dropped back.
"It was just a case of trying to fight my way to the front.
"I was quite a bit behind and just picking them off one at a time. I just wish I was a bit closer to be in the race, but I'm really happy from where I came from to get up to fifth."
Fogg had edged out Davies in Portugal in June to qualify as the only British swimmer in the race. Around the halfway point, it looked as if his decision to combine the event with the 1500m freestyle, where he finished eighth in Sunday's final, had backfired.
As Germany's Andreas Waschberger raised the pace and strung out the field on the fourth of sixth laps, Fogg slipped back to 22nd in the field.
But with Mellouli forging clear at the front and the pace wearing down the open water specialists around him, Fogg used his speed from the shorter course to clamber up the standings almost unnoticed.
The 24-year-old Londoner finished just 42 seconds off the winner, following home world champion Spyridon Giannotis in fourth.
Giannotis, like the rest of the open water established order, could not muster a response to the audacious break from Mellouli, who assured a unique swimming double in his fourth appearance at the Games.
"I can't explain it, I can't really describe it," Mellouli said, pumping his chest after claiming victory. "I don't think this has ever been done before. This is probably one of the toughest things to do.
"I've been struggling, with my shoulder, my elbow, I had a virus. What happened today is a miracle, if you believe in miracles.
"This thing just hurts. You're in pain. Once you hit a wall you just keep pushing, when you hit a wall again you keep pushing."