The much-hyped showdown between superfish Michael Phelps and Grant Hackett in the 400m freestyle is over before it even started.

The American flopped on the opening morning of the world championships, failing to make the final by badly fading in his heat at the Parc Jean-Drapeau pool in Montreal.

The Baltimore Bullet was off target in his first international outing in the eight-lap race, coming seventh in his heat and 18th in qualifying in a time of three minutes 50.53 seconds.

The star of the pool at last year's Athens Olympics seemed as stunned by his poor showing as the rest of the swimming world.

"I felt good in the warm up, but in the race it just wasn't there," said a clearly rattled Phelps who was almost four seconds off his best time.

"I have a relay tonight and I have to put this behind me and step up for my other events.

"This is not how I wanted to start."

Meanwhile Hackett cruised through to the final with his best time in a 400m heat, touching the wall in 3:44.63.

The team captain was shocked not to be lining up next to Phelps in the final after competing in the next heat after the six-time Olympic gold medallist.

"I was awfully surprised and spent the first 200 metres of the race thinking about it, it was that much of a shock," said Hackett, who is a hot favourite to win on Monday night.

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