Belgrade - Novak Djokovic was forced to withdraw from the opening day of the Davis Cup World Group semi-final between Serbia and Czech Republic on Friday due to stomach problems, a statement said.
Victor Troicki, world ranked 47, will replace Djokovic in the first rubber against Czech Radek Stepanek, ranked 35th, a statement on the Davis Cup website said.
It was unclear whether Serbia's top player, who had only returned from US Open early on Wednesday where he lost in the final to Rafael Nadal, would be able to play in a singles match on Sunday.
Serbia's Tennis Association officials were not available for comment.
Serbia's team was impressive in the Davis Cup this season, defeating the United States 3-2 in Belgrade and then cruising past Croatia 4-1 in the quarter-finals in Split.