Davenport, who missed last week's Hopman Cup with a knee problem, said she had been feeling ill for a few days but it wasn't until she woke up yesterday that she realized something was wrong.
"I was actually feeling a little bit better yesterday but when I woke up this morning I felt worse. I was struggling for energy and catching my breath," the Californian said.
"It was just a little concerning because it's been getting worse over the last five days and at some point you've got to stop. I just can't keep running myself into the ground."
Davenport, seeded first for the Australian Open, an event she won in 2000, said she was taking medication and would try and get as much rest as possible in order to be ready for next week.
"Antibiotics and rest, that's all that will help," she said.
"I normally like to play a lot before a grand slam but sometimes you're just dealt with the hand you're given. I'm thankful I have a few more days for it to hopefully clear but I haven't thought about it too much. It's just one of those things that's really bad luck and I've just got to move forward," she said.
Hewitt still on track
Australia's Lleyton Hewitt kept his cool on a blistering hot summer's day to reach the semifinals of the Sydney International yesterday while his main rivals all melted.
The world number three breezed into the last four of the Australian Open warm-up event with a 6-1 7-6 win over Swede Thomas Johansson, to be the last remaining seed in the men's draw.
Fourth seed Romanian Andrei Pavel, Spanish fifth seed Feliciano Lopez and Taylor Dent, the big-serving American seeded eighth, all succumbed to lower ranked opponents as the temperature at Sydney's Olympic tennis center soared to 42?C.
Pavel fell to little-known Czech qualifier Ivo Minar 2-6 7-6 6-4, while Lopez succumbed to Radek Stepanek 6-3 6-2.
Dent, who beat Hewitt in last week's Australian hardcourt championships, failed to finish his quarterfinal with Max Mirnyi of Belarus, quitting halfway through the first set after feeling ill.
In the women's draw, Russia's world number 13 Nadia Petrova quit her match against Chinese amateur Peng Shuai because of heatstroke. The fifth seed was trailing 6-3 4-2 at the time.
Of the five Russian women who entered the event, only third seed Elena Dementieva remains in the draw. The French and US Open finalist beat Swiss Patty Schnyder 6-4 1-6 6-1 to set up a possible semifinal with world No. 1 Lindsay Davenport.
Peng will play Alicia Molik in the other semifinal after the rapidly-improving Australian beat Colombia's Fabiola Zuluaga 6-1 6-2.
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