Germany's top tennis player, Tommy Haas, will fly to the United States today for toxicological tests on his hair after bizarre allegations that he was poisoned during his country's Davis Cup defeat to Russia in Moscow two months ago.

The International Tennis Federation is also investigating the claims from Alexander Waske, Haas's Davis Cup teammate, who said that he had been told of the poisoning by a Russian manager.

Haas, who is ranked 13th in the world, played on the opening day of the tie on September 21 but lost in straight sets – 6-2, 6-2, 6-2 – to Igor Andreev, ranked 33.

The Germans turned the contest around and went into the final day 2-1 ahead but Haas was unable to take to the court on the Sunday for his reverse singles clash with Mikhail Youzhny due to stomach pains – and his replacement Philipp Petzschner went down in four sets.

Russia then clinched the decider with Andreev beating Philipp Kohlschreiber.

Waske, a doubles specialist, said that he was told "completely casually" by a Russian manager – whom he did not name, but who manages many players – that Haas had been deliberately nobbled.

“He said as an aside, that it was a shame that Tommy was poisoned,” said Waske, who answered the man by saying that it was a virus. “Thereupon he said, no, they poisoned him.”

When Waske expressed his scepticism at the claim his informer reportedly told him: "Believe me Alex, I know Moscow, there are people who can make these kinds of things happen."

If Waske's claims are proven, they will join a list of extraordinary Russian poisoning sagas, including the murder in London a year ago of the former KGB officer Alexander Litvinenko, who was fed a radioactive isotope in a cup of tea at a Mayfair hotel. Britain is demanding the extradition of another former KGB officer, Andrei Lugovoy.

The other most high-profile poisoning allegation came from the Ukrainian President Viktor Yushchenko, who almost died after ingesting dioxin while campaigning for the presidency on an anti-Moscow platform.

The International Tennis Federation, the sport's governing body, said that it was launching an immediate investigation into the Tommy Haas allegations, to be headed by Bill Babcock, president of its Davis Cup committee.

Haas, 29, said that he would fly today from Argentina to his home in the United States for a complete check-up. The German team doctor Erich Rembeck had 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, who is ranked 13th in the world. “When I think of how bad I felt, I can imagine it. I’ve been feeling weak for weeks.”

He added: “I have never felt so miserable in my whole life as I did on the Saturday and Sunday nights in Moscow. Of the eight hours I should have been sleeping, I spent six of them on the toilet."

Six weeks after the alleged incident, Haas said that he was still feeling the effects. "My stomach is still doing some strange things."

Rembeck did not rule out that someone had tampered with Haas’s 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.”