(CNN) -- World number one Roger Federer is suffering from a lung infection and has pulled out of this week's $2 million ATP tournament in Dubai.
Australian Open champion Federer is set to be sidelined for up to six weeks and may not return to action until a big hard court event in the United States in March.
Federer, who won his 16th grand slam when beating Britain's Andy Murray in Melbourne, has been advised by a doctor not to play for at least a fortnight.
He is set to play at the Masters Series event at Indian Wells which starts on March 11, but may have to wait until the following tournament in Miami.
"There are no guarantees that I will play at Indian Wells -- this could be a maximum of six weeks, but if it's treated properly everything should be fine and I should be back fairly quickly," he told the official ATP Tour Web site.
Federer, who is a four-time Dubai Open champion, had traveled to the United Arab Emirates to practice ahead of the tournament, but found he was struggling.
"On Wednesday I couldn't get up anymore - and the next day I just wasn't in shape at all," he said.
"It's a lung infection -- it's the first time I've had it - breathing is difficult -- I feel I am not fine, I'm still very tired and slow.
"The doctors say it's too serious for me to try -- and that I should rest for at least two weeks -- obviously I can't take any more chances."
Federer, who has a home in Dubai, was looking to regain his title from Novak Djokovic, who will now be top seed, with Murray also in the field for his first tournament since Australia.
Czech Jan Hernych, who went out in the final round of qualifying, benefits from Federer's withdrawal to go into the main draw.
In men's ATP action on Sunday, Sam Querrey beat fellow American John Isner in the final of the tournament in Memphis.
Eighth seed Querrey, who had accounted for Andy Roddick in the quarterfinals, beat Isner, seeded sixth, 6-7 7-6 6-3 in a closely-contested match.