Roger Federer is uncertain if he will recover from a back injury in time to play for Switzerland in the Davis Cup final, according to Novak Djokovic.
Federer pulled out of his title showdown with Djokovic at the ATP Tour finals on Sunday just before the match was due to begin.
The world number two now faces a race against time to be fit for the Davis Cup final against France, which gets underway in Lille on Friday.
The 33-year-old aggravated a back problem, which has troubled him at various times this year, during a gruelling three-set victory over Davis Cup team-mate Stan Wawrinka on Friday.
Djokovic spoke to Federer after learning of his rival's withdrawal and said he told him it will be a battle to make the first Davis Cup final of his illustrious career.
"I spoke to him and it's a question mark for the Davis Cup final. He doesn't know yet," said Djokovic.
"The stats are saying clearly that he is one of those players that would compete and would always fight if he can. He has had, what, three retirements in his entire career.
"I don't think he was calculating and trying to save his body for Davis Cup final.
"I'm sure that wasn't the case. This is probably one of the biggest matches of the year for him. I'm sure if he could have, he would be on the court."
Federer's withdrawal made Djokovic the Tour Finals champion for the third successive year.
The world number one is the first player to complete a hat-trick at the prestigious season-ending event since Ivan Lendl in the 1980s, but he admitted it was a strange way to end his season.
"During the doubles match, that's when I found out. I did my warm-up, obviously prepared for the battle, prepared for a great match as it was anticipated, as the people expected it to be," Djokovic said.
"This is probably the first time in history that this happens. It's a very awkward situation to talk about it, to be honest, today.
"You never like to win, especially these big matches against big rivals, with the retirement. But that's the way it is."
Federer admitted that currently he was in no condition to play.
Worryingly for Switzerland's hopes of a Davis Cup win next weekend is that the tie is being played on clay, the most gruelling of the sport's surfaces.
"Unfortunately I'm not match fit to play match tonight. Clearly I wish it wasn't this way. You try all year to be ready for the ATP World Tour Finals and I didn't want to end this way," said Federer.
"I tried everything I could last night and today -- painkillers, rest -- until the very end, but I can't compete at this level with Novak.
"In a final like this and at my age, it would be too risky. I hope you understand."