The relationship between Swiss tennis legends Roger Federer and Stanislas Wawrinka might no longer be the same again following reports of a heated backstage confrontation in the wake of their semifinal clash at the ATP Tour Finals at O2 Arena in London.

According to a source close to situation, the 33-year old Federer traded barbs with Wawrinka to the point that tournament organizers forced them to settle their score inside a locker room.

ESPN Tennis analyst and former Grand Slam champion John McEnroe revealed the incident that happened between Federer and Wawrinka, adding that this rift could put the Swiss' Davis Cup title aspiration in jeopardy.

"Something went on in the locker-room, there was a long talk between the players that extended well into the night," McEnroe said during a panel discussion on ESPN. And the stress of that -- I can't confirm all of this -- but a lot of this went on and I don't think that helped the situation."

Sources indicated that Federer's wife, Mirka, triggered the rift between Roger and Stan. Mirka allegedly made an audible comment from Federer's players box, asking Wawrinka in French if he has the guts to make a rally against her husband.

"Among Wawrinka's complaints is that she made audible comments from his supporters' box - which unlike in many stadiums are right at ground level - questioning in French whether he would have the guts to close the match out," according to the Daily Mail.

Federer went on to win the game against Wawrinka 4-6 7-5 7-6, but the 17-time Grand Slam winner surprisingly withdrew from his final match with Novak Djokovic due to back spasms.

While his latest back injury is concerning, Federer's current relationship with Wawrinka is a far more serious problem for the Swiss Davis Cup team as they take on France for the championship starting on Friday. Federer and Wawrinka could be slated to play together in the crucial men's doubles match on the second day at Stade Pierre-Mauroy in Lille. Federer is seeking his first Davis Cup win in his career.