Federer said in a statement on his Web site Wednesday that he had been playing catch-up all year after being diagnosed with mononucleosis at the start of 2008.
"I feel fortunate to be healthy again, but I want to remain at the top of the game for many more years to come and go after the No. 1 ranking again,'' Federer said. "In order to do that, I need to get a proper rest and get strong again so that I am 100 percent fit for the remainder of the year or next year. At this point, I am not sure when I will be ready to play again, but I hope to be back at some point before the end of the year.''
Federer apologized to fans who had hoped to see him play in the tournament, which starts Monday at Stockholm's Royal Tennis Hall.
The second-ranked Federer lost the No. 1 spot to Rafael Nadal this year after the Spaniard beat his Swiss rival in both the French Open and Wimbledon final.
Stockholm Open officials confirmed Federer had withdrawn from the tournament on Wednesday.
"We are very sad and disappointed,'' tournament director Per Hjertquist said. "We're looking for a replacement. But who can replace Federer?''
He said Nadal had already declined an invitation to play in Stockholm, leaving No. 7 David Nalbandian as the top-ranked player at the tournament.
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