World number one Roger Federer [Images] has withdrawn from the Kooyong Classic, the warm-up event for next week's Australian Open, because of a stomach virus.

In a statement, the Swiss said he had to miss the event in order to recover in time for the defence of his Australian Open title at Melbourne Park.

"After arriving in Australia last Friday, I came down with a terrible stomach virus on Saturday and it continued to get worse over the weekend," Federer said.

"After trying to practice on Saturday afternoon, I went to see the doctors and they immediately performed a series of tests. The doctors have advised me that I should take a couple of days off and recuperate so that I am able to be 100 percent for the start of the Australian Open.

I hope to resume practice by the middle of the week."

A replacement for Federer has yet to be named.

German Tommy Haas also withdrew from the eight-man exhibition event on Monday, having failed to recover in time after recent shoulder surgery.

The former world number two, who still hopes to be fit for the Australian Open, will be replaced by Russian world number four Nikolay Davydenko in the Kooyong event.