Australian Ky Hurst had no excuses and no idea what went wrong as he bowed out of open water swimming with a disappointing 20th placing in the Olympic 10km marathon in London.
The surf ironman great had come to his second Games with high hopes after finishing fifth at last year's world championships but he faded badly during the late stages at The Serpentine in Hyde Park.
Tunisian Oussama Mellouli won the gold medal to become the first man to win Olympic gold in both open water and the pool, after he defeated Grant Hackett to win the 1500m freestyle title in Beijing four years ago.
Mellouli, who took bronze in the 1500m in London last week, clocked one hour, 49 minutes and 55.1 seconds to fend off German Thomas Lurz (1:49:58.5) and Canada's Richard Weinberger (1:50:00.3)
Hurst, 31, was fifth after two of six laps but was left scratching his head after dropping back to finish 20th in 1:51:41.3.
"I'm a bit confused by it all," Hurst said.
"My preparation leading into this has been awesome, I'd probably been swimming better than I ever had before.
"... I've got no excuses , I'm just a little bit shocked that I couldn't back that last half up. It's usually the better half of my race."
Hurst will now retire from open water swimming to focus on the ironman series, in which he is a four-time national champion.
While he hoped to sign off in better fashion, he had no regrets about pushing for a second Games.
"I've had a great career and this has been fantastic, the experience," said Hurst, who finished 11th in Beijing.
"... I feel really blessed I was a part of this."
Mellouli's victory represented yet another milestone for the pioneering 28-year-old.
He became the first African swimmer to win an Olympic gold when he beat Hackett in Beijing, three months after returning from an 18-month doping ban.
He made the early running on a warm London afternoon and, despite dropping back slightly, remained in the leading pack throughout.
He took the initiative once again on the sixth lap and extended his lead to 13 seconds at one point, but he was fading fast in the final sprint and had to grit his teeth to beat Beijing bronze medallist Lurz to the finishing line.