Haas was forced out of his match against Mikhail Youzhny with a suspected stomach virus as Russia won both reverse singles matches on Sept. 23 to win the semifinal series 3-2 and reach the Davis Cup final.
"We take this very seriously," ITF spokeswoman Barbara Travers said Wednesday. "The investigation starts today."
German teammate Alexander Waske said he was told by a Russian who manages numerous athletes that it was poisoning, not a virus. Waske didn't say who the manager was.
"He said as an aside, that it was bitter that Tommy Haas was poisoned," said Waske, who answered the man by saying that it was a virus. "Thereupon he said, no, they poisoned him."
Haas said he will fly Thursday from Argentina to his home in the United States for a complete checkup. Germany team doctor Erich Rembeck had done a checkup and run blood tests while the player was sick, but said they were only for a virus.
"I'm shocked that something like that appears possible," said Haas, ranked 13th. "When I think of how bad I felt, I can imagine it. I've been feeling weak for weeks."
Rembeck didn't rule out tampering with Haas' food or drinks. He said the German staff drew up meal plans, but the kitchen staff at the hotel prepared what ended up on the players' plates.
"I was the only one ever to order dessert or a Latte macchiato after dinner," Haas said. "If all this is true, since no one else got sick, that must have been when it happened."