"I did a test because I suspected that I had swine flu. The test was positive," Haas told the Bild newspaper. "Of course, it was a shock for me."
Haas told the paper he was already doing better.
"I've calmed down now because although swine flu is a very strong form of flu, you can quickly overcome it when you're in good physical condition like I am," Haas was quoted as saying in Tuesday's edition of the newspaper.
The 31-year-old Haas, who is ranked 17th in the world, pulled out of a tournament in Stockholm last week after becoming ill. He first thought it was normal flu but then had himself tested.
He also has pulled out of this week's Swiss Indoors and is unlikely to play next week at the Paris Masters.
German gymnast Philipp Boy and handball player Dominik Klein have also had swine flu.