(CNN) -- German tennis star Tommy Haas has been gone down with swine flu and is set to miss key end of season tournaments.
The 31-year-old pulled out of last week's event in Stockholm, feeling unwell with flu-like symptoms.
"I did a test because I suspected that I had swine flu. The test was positive," Haas was quoted on the Web site of the Bild newspaper.
"Of course, it was a shock for me."
Haas is hoping to recover quickly to resume a season which has seen him rediscover his top form, reaching the semifinals at Wimbledon and winning his 12th ATP Tour title on the grass of Halle.
"When you are someone who is as physically fit a I am you can quite easily get over this flu," he said.
But he has been forced to withdraw from this week's big indoor tournament in Basel, which is headed by world number one Roger Federer.
Haas could also miss the Paris indoor event as he bids to improve his world ranking of 17.
Federer, who has had a six-week break, made a winning return on Monday night in his quest for a fourth straight title in his home city.
The Swiss maestro beat Belgium's Olivier Rochus 6-3 6-4 and will next play 51st-ranked Andreas Seppi of Italy, who beat Swiss wild-card entry Stephane Bohli in three sets.
Federer had not played since helping Switzerland defeat Italy in the Davis Cup Wordl Group playoffs on September 20.
Second-seeded Serbian Novak Djokovic began his campaign on Tuesday with a straight sets 6-3 7-5 win against Germany's Andreas Beck.
Djokovic needed 90 minutes to set up a second-round match with Czech Jan Hernych, who beat Australian qualifier Peter Luczak 6-3 6-4.