(CNN) -- Beaten U.S. Open finalist Novak Djokovic has been forced to withdraw from Serbia's Davis Cup semifinal against Czech Republic with gastroenteritis.
The world number two was set to face Radek Stepanek in Friday's opening rubber in Belgrade, but was ruled out after coming down with the stomach illness.
The 23-year-old has been replaced in the singles line-up by the 24-year-old Viktor Troicki, who is ranked 47th in the world.
Elsewhere, the man Djokovic beat in the semifinals at Flushing Meadows, Roger Federer, opted against playing for Switzerland in their world group play-off against Kazahkstan.
The 16-time grand slam champion cited his exertions at the New York tournament as the reason behind his decision.
The Swiss maestro said on his official website: "I have decided not to join the Davis Cup team to Kazakhstan.
"I need some extra time to relax after the intense weeks in North America so I can finish the year strong."