The International Tennis Federation is investigating claims Germany's Tommy Haas was poisoned during the Davis Cup semi-final defeat by Russia.
Team-mate Alexander Waske told German tabloid Bild that an unnamed Russian suggested to him Haas had been poisoned in Moscow.
"We take this very seriously," said an ITF spokeswoman.
On Thursday the German tennis federation said there was no medical evidence to support the speculation.
Haas, however, will travel to New York on Friday to undergo hair and blood tests to determine whether there is any truth in the claims.
The 29-year-old lost the opening singles match in straight sets and was unable to play again due to a case of gastro-enteritis.
"I want to find out if any poison can be traced"
Germany went into the final day with a 2-1 lead but Haas' replacement Philipp Petzschner lost in four sets before Russia took the decider with Igor Andreev beating Philipp Kohlschreiber.
Waske said the poisoning claim was made by a Russian who manages various athletes.
"I spoke with a Russian from Moscow at the Davis Cup," said Waske. "He said completely casually that Haas had been poisoned.
Former world number two Haas told Bild: "I was shocked when Alex told me this but when I thought back about how bad I felt at the time I can imagine it's possible.
"I sat - or better laid - six hours on the toilet. I have never felt so miserable in my life - I was really scared.
"I want to find out if any poison can be traced or confirmed."
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A DTB spokesman said the German team doctor Erich Rembeck had noted a lack of evidence to substantiate the rumours.
"There is no medical reason that justifies further, targeted tests with regard to poisoning," the spokesman said. "As an association, we must rely on facts in judging and assessing the situation and not on speculation.
"Therefore, because we have no evidence of poisoning and because we do not know how reliable the informant is, we must assume that it's no more than speculation."
He added: "We lost at the weekend in question because the Russian team was better during those three days.
"We are not thinking about filing a protest about the result of the Davis Cup match."
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