Ian Thorpe avoided Michael Phelps at the Qantas Skins.

The world's best swimmers were supposed to anchor the Australian and American 50m relay teams in their only head-to-head clash but Thorpe was a late withdrawal because of fatigue.

The result of their showdown would have been insignificant in the grand scheme of things, of course, but more than 6,000 supporters at the Sydney Olympic Park Aquatic Centre and a sizeable live television audience were still hugely deflated by the Olympic champion's decision.

Any clash between Thorpe - who arrived back from a three-week training camp at altitude in Flagstaff, Arizona - and the man threatening to overtake him as the world's premier swimmer is of genuine intrigue.

"I'm weary after the training and the flight home was tough," said Thorpe.

"I'm not feeling all that good, not feeling 100 per cent. It's pretty tough. Having a flight like that, coming back down to sea level, trying to make all the adjustments to the time, really trying to rush back to get here tonight ... it's really taken a toll on my body.

"It took about a full day to get home. Getting up to race is tough."

Asked if he was trying to build the hype surrounding him and Phelps by avoiding the 18-year-old all the way through to next year's Olympics, Thorpe said: "No, not at all. I'm not into doing that. No."

Thorpe withdrew from a planned training stint with Phelps earlier this year, which the American apparently wasn't too happy about.

Thorpe clocked a personal best time of 24.2 seconds tonight to win the 50m freestyle, adding $5,000 to his bulging bank account.

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