Grant Hackett held off the biggest challenge to his eight-year reign as the world's distance swimming king, winning back-to-back Olympic 1500m freestyle titles.

Hackett clocked the fourth fastest 1500m freestyle in history to beat teenagers, American Larsen Jensen and Briton David Davies, who have finally breathed new life into the gruelling 30-lap race.

The 24-year-old world recordholder won the gold in 14 minutes 43.40 seconds, less than two seconds ahead of Jensen (14:45.29) and was so exhausted after the race he simply held his finger towards the camera indicating "number one".

Minutes after the race, he climbed onto his block and held his arms in the air as his Australian teammates got to their feet and hailed him as the king of the event.

Hackett's time was about 10 seconds outside his world record of 14:34.56, set at the 2001 world championships in Fukuoka, Japan.

Jensen and Davies (14:45.95), who both turn 19 this year, are only the third and fourth swimmers to break 14:50 for the distance behind Kieren Perkins and Hackett.

"They're not making the next four years any easier right now," Hackett said.

Jensen said he went into the race believing no-one was invincible.

"If there's something I've learnt here, especially at these Olympic Games, it is that anyone is beatable," he said.

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