Roger Federer finds himself the subject of much speculation following his withdrawal from the final of the ATP World Tour Finals with a back injury.

Federer had played a long three-setter with Swiss compatriot Stan Wawrinka the night before, and has the Davis Cup final to play this week alongside Wawrinka. But reports of conflict with Wawrinka and questions about whether Federer will be fit enough for Davis Cup have swirled since the 17-time Grand Slam champion announced he would not be able to play Novak Djokovic for the season-ending title.

Photographers sitting courtside during the match are reported to have heard Wawrinka directing remarks at Federer's wife Mirka, apparently due to her cheering. Wawrinka appeared to object to the timing of the cheers, with French television said to have heard the French-speaking Wawrinka saying, "She also did it at Wimbledon." Wawrinka and Federer played in the quarterfinals at Wimbledon.

John McEnroe, commentating for ESPN2, referred to "goings on" between the two and said there had been a discussion between the players following the match.

"Something went on in the locker room, there was a long talk between the players that extended well into the night," McEnroe said. "And the stress of that—I can’t confirm all of this—but... I don’t think that helped."

Federer gave an on-court speech and a TV interview to the ATP (WATCH HERE), but declined to give a press conference following his withdrawal. He spent much of 2013 dealing with a back injury and has experienced periodic back problems throughout his career. In an ATP interview, Federer indicated that he still expected to play Davis Cup, referring to "traveling to France at some point and getting ready on clay for the Davis Cup final."

The tie is a highly anticipated one, with neither Switzerland or Federer ever having won the event. France will field a deep team that includes Jo-Wilfried Tsonga, Richard Gasquet, and Gael Monfils.

Novak Djokovic, however, said Federer had been circumspect about his participation. "I spoke to him," Djokovic said, having played an exhibition against Andy Murray instead of the scheduled final. "It's a question mark for Davis Cup as well."