Eva Risztov of Hungary held on to win the gold medal for the women's marathon swim at the London Olympics after a desperate sprint finish to the 10-kilometre race.
Haley Anderson of the United States won the silver medal after making a late charge to catch Risztov while Italy's Martina Grimaldi won bronze.
"It was a really tough race but I was prepared," said Risztov.
"In a few hours I will be much more happy because I'm really, really, really tired now."
Australia's Melissa Gorman swam with the lead pack for much of the race before finishing 11th, more than a minute off the pace.
Gorman said the race did not go to plan.
"I would have liked to have done a lot better than that. I would have liked to come away with a medal today and I think I was capable of it," she said.
"I gave it everything I had and that's all you can do.
"I would have liked to have stayed with the front pack through the race.
"It was a rough race out there, everyone was bouncing and once I took a few hits, I struggled to stay with the leading group."
Britain's Keri-Anne Payne, who won the gruelling event at the last two world championships, finished fourth from the 25 starters at the Serpentine, a recreational lake in Hyde Park.
"It was a pretty violent race," said Payne, who lost touch with the leaders when she paused to get an energy drink after the third of the six loops.
"I had to work pretty hard to get back to the position I like. I'm not really a fighter, I'm more of a lover, I guess."
A former Olympic pool swimmer who quit the sport in 2005 then made a comeback in open water in 2009, Risztov led for all but one of the six loops the competitors were required to swim.
She opened up a gap on the leading pack at the start of the final loop and was three body lengths clear approaching the finish when Anderson started to reel her in.
Anderson, whose older sister Alyssa won gold as part of the American 4x200m freestyle relay team, drew level with Risztov as they approached the finish but the Hungarian kicked again and got her hands on the pads first in one hour 57 minutes and 38.2 seconds, just four tenths of a second in front of Anderson.
"I tried to catch up and I'm just happy with winning a medal," said Anderson.
"It takes a lot of mental confidence. It's really hard but really rewarding."
Grimaldi, runner-up at last year's world championships in Shanghai, won bronze by the same tiny margin ahead of Payne.
The exhausting race took its toll on two competitors, South Africa's Jessica Roux and Brazil's Poliana Okimoto, who were both hauled from the water before the end.
Tens of thousands of people lined the banks of the lake on a glorious summer's day for one of the few events at the Games that were free for the public to watch.