Haas, 31, also pulled out of the Stockholm Open last month because of illness. He said he then decided to be tested and was diagnosed with having the H1N1 variant of the flu.
"It was really bad for three days and I couldn't leave my hotel room," Haas told the German newspaper Bild.
"I am quite relaxed because I know that swine flu is a particularly strong form of flu but when you are in good physical conditions like I am, then you can quickly beat it off."
Haas said he was feeling better this week but has withdrawn from next week's Master's tournament in Paris.
Haas is ranked No. 17 in the world and has won one tournament -- the event in Halle, Germany, in June -- this year. He has 12 ATP titles over a 13-year professional career.