Michael Phelps [Images] shattered his own world record in the 400 meters individual medley final on Sunday in a devastating start to his campaign to collect eight Olympic gold medals in China.

The lanky American fought off a strong challenge from Hungary's Laszlo Cseh and US team mate Ryan Lochte to win the gold in four minutes 03.84 seconds, wiping 1.41 off the world record of 4:05.25 he set at the US Olympic trials six weeks ago.

"I'm pretty happy with that, it was a pretty emotional race," said Phelps, who was cheered on from the stands by US President George W Bush [Images].

"I looked up and he waved the flag, nodded his head, that was pretty cool, a pretty cool feeling to have the president say congratulations and be here today."

Cseh, who set a cracking pace to lead the race after the first butterfly lap, slipped back to third after the breaststroke then fought back strongly to snatch the silver in 4:06.16 while Lochte won bronze in 4:08.09 after fading on the concluding freestyle leg after briefly threatening Phelps.

"I knew it was going to be tough all the way through," said Phelps.

"Looking and seeing all three of us close together at the 200 I wasn't really very comfortable, I wanted to have more of a gap.

"I made the breaststroke stronger. The freestyle was all downhill, it was all adrenaline."

Phelps, 23, won six gold medals at the last Olympics [Images] in Athens but is chasing eight in Beijing [Images] in an ambitious bid to break Mark Spitz's [Images] record of seven from Munich in 1972.

The odds are stacked against him but the manner and authority of his victory on Sunday suggests he is the form to make a serious challenge at a record many never thought would be broken.

The 400 individual medley was regarded as one of his toughest races after Lochte came close to beating him at the US trials, with both men going under Phelps' old world record.

"It wasn't my best time I'm not really proud of it but I did everything I could, that's all I can ask for," Lochte said.

"My backstroke leg hurt more than I wanted it to. The breaststroke I held on the freestyle I was dead.

"[Phelps] was amazing, he's taken a world record, you can't ask for anything else. It was a great way to start a meet for the USA."