Serbia's Janko Tipsarevic is a doubt for this weekend's Davis Cup final against the Czech Republic in Belgrade.
The Serbs are hot favourites to win the title - they are playing at home and have world No 2 Novak Djokovic - in the form of his life - in their ranks.
But the loss of Tipsarevic will pile further pressure on Djokovic to deliver two singles victories and one in the doubles where he is expected to team up with veteran doubles specialist Nenad Zimonjic.
Tipsarevic - the world No 36 - says he is struggling to recover from a chronic heel injury in time for the showdown on the indoor hard courts of the Belgrade Arena.
A final decision on whether he will play will be made before Thursday's draw.
If he is unavailable, the Serbs' other options are 117th-ranked Dusan Lajovic, who has played one dead rubber, 227th-ranked Filip Krajinovic, yet to play in the cup, or doubles specialist Ilija Bozoljac, who has played four cup singles, all of them dead rubbers.
The Czechs will have Tomas Berdych, the world No 7 and the wily veteran Radek Stepanek and both would expect to beat Bozoljac, making the doubles clash on Saturday pivotal to the outcome of the match.
"Doubles is definitely the most important point," Berdych said. "It happened for us many times in the past."
Djokovic said that although he has not played many doubles in recent years, he would be ready on Saturday if needed.
"It's a different tactical approach between doubles and singles," Djokovic said, adding that the decision whether he would play doubles depends on the result of the first-day singles on Friday.
Serbia won the title for the first and only time at the same Belgrade Arena three years ago and the hosts are sure to enjoy rousing support from the 15,500 home fans.
The 26-year-old Djokovic is on a 22-match winning streak, having won the last four tournaments he has contested in Beijing, Shanghai, Paris and London.
Big-hitting Berdych has struggled in match-ups with Djokovic in the past, winning just two of their 16 encounters.
He has also lost all 12 of his meetings with the Serb on a hard court.
Djokovic said: "Tomas is one of the best players in the world and although he's never beaten me on a hard court, we have had a lot of long and tough matches."
Berdych added: "It's not going to be easy at all. But we are in a position that this is not a single tournament - we have five matches to try to score three points.
"Basically every match is very important but if we don't win some of them we still just need three points - we don't need all of them."