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Video - Mellouli's 1500m medal win
David Davies suffered Olympic 1500m freestyle final heartbreak as the Welsh swimmer finished outside the medals.
Davies hoped to better his Olympic bronze of 2004 but in Sunday's final in the Water Cube on Sunday, the 23-year-old was off the gruelling pace.
Tunisia's Oussama Mellouli won to stun double Olympic champion Grant Hackett.
Davies said: "The elephants and pianos were all on my shoulders, my heart was pounding and my stomach gone. It's a long way if you're feeling like that."
Hackett, who was going for his third successive Olympic 1500m gold in Beijing, led for much of the race before Mellouli staged a late charge with 200m to go.
Mellouli held on to win Tunisia's first ever Olympic swimming gold medal in a time of 14 minutes 40.84 seconds.
Canadian Ryan Cochrane claimed the bronze with Commonwealth champion Davies sixth in a time of 14 minutes 52.11 seconds, exactly six seconds slower than his qualifying time in Friday's heat.
The times swam here in Beijing were like nothing I have experienced before
"I knew the 36 hours in between would be a race to recover and I did everything I could, but in the end it was really telling on me and it caught me out," admitted Davies.
"The times swam here in Beijing were like nothing I have experienced before, it was incredibly tough just to qualify for the final.
"To qualify for the final I had to swim my best time for four years and I'm impressed with those guys who clocked those kind of times in the final just 36 hours after a huge hit in qualifying.
"Normally you can stroll through the heats but I had to swim a world-class time just to get through and it took its toll in the back end of that final, I tried to kick but the tank was empty.
"I tried my best but sadly I did not have the recovery powers.
"The event has changed and everybody is going to have to re-think their strategies, this is now basically a 1500m sprint - it's unbelievable.
"Now if I want to carry on in this event for another four years, I've got to find a way to do maxed out 1500m in a short space of time if I want to compete.
"I would have liked to have gone quicker but I've got to recover for the 10k, that will be a different challenge and a tough test and I'm looking forward to it."
Mellouli, 24, returned from an 18-month doping ban in May in time to swim at the Olympics.
"I've been waiting for this moment for two years," he said after his 1500m victory .
"I had expectations for the 400m freestyle but this is a great surprise.
"I felt good in the first 400m of this race and at the 800m and 900m I started believing that I could win.
"It was all calculated. I slowed down in the penultimate 100m to save my energy and attack in the final 50m."
After suffering defeat Hackett said he was undecided about his swimming future beyond Beijing, even hinting at retirement.
He said: "I'm not sure. I'm a little older now, I'll see what I feel like, have a break, then see what happens."
No man has ever won three consecutive Olympic titles in the same event, although Australian sprinter Dawn Fraser and Hungarian backstroker Krisztina Egerszegi achieved the feat in women's swimming.
"It's disappointing, yet so close," Hackett said. "To get second is great, but three in a row would have been nice.
"I have certainly no regrets in my preparation and what I've been able to do here. It was certainly a good race.
"I gave it everything today, it was a hard race. I'm a little bit tired. The heat swim was the second fastest 1500m I have ever done."
Davies concluded: "I'm not sure what the future is for Grant Hackett but what he chooses, he has been a wonderful ambassador to this sport and it was my privilege to race against him.
"To be within a shot of winning three Olympic titles in this the hardest and most gruelling swimming event of the lot was an amazing achievement.
"I feel gutted for Grant here, I would have really liked to see him win it."