Switzerland's Davis Cup hopes lie on the precipice of ruin after relations between an injured Roger Federer and Stanislas Wawrinka were damaged following talks after their epic three-set semi-final at the ATP World Tour Finals which are understood to have further accentuated the 17-time major champion's back problem.
The 33 year old withdrew from the climax to the ATP season just hours before facing world No.1 Novak Djokovic with a recurrence of a back injury suffered during the third-set tiebreak against Wawrinka on Saturday evening.
The walkover hands Djokovic a third straight title at the World Tour Finals, but the Serbian's feat was overshadowed by what is being suggested as infighting within the Swiss team less than a week before they face France in pursuit of a first ever Davis Cup title.
The world No.2 Federer saved four match points before eventually prevailing 4-6 7-5 7-6(6) in two hours and 48 minutes over his Davis Cup teammate, with the match concluding at around 11pm local time.
Federer's various media commitments with television, radio and the written press eventually concluded at 1am, just 17 hours before his scheduled meeting with Djokovic in Sunday's final.
But between securing passage to an eighth final at the season-ending tour championships and completing his media obligations, Federer held clear-the-air talks with Wawrinka following a heated exchange between the 29 year old and members of his opponents' players box.
Late in the third set, Wawrinka directed a tirade of abuse towards the box which contained Federer's support team, including coach Stefan Edberg and wife Mirka, after noise was made just before he received serve.
Wawrinka was overheard by French television muttering: "She did same thing at Wimbledon," where the Australian Open champion won the first set before losing 3-6 7-6(5) 6-4 6-4 in the quarter-finals.
The brief confrontation is understood to have been discussed between both players in the locker room after the match, delaying treatment on Federer's back injury, which was aggravated during the talks and led to his eventual withdrawal.
The seriousness of Federer's injury and shock behind him pulling out is amplified by the statistic that he has previously withdrawn from just two of his previous 1221 matches.
After reports engulfed the O2 Arena regarding the late-night discussions, seven-time grand slam champion John McEnroe told ESPN: "Afterwards, something went on in the locker room, there was a long talk between the players that extended well into the night.
"And the stress of that - I can't confirm all of this - but a lot of this went on and that caused....I don't think that helped the situation.
"That makes [the Davis Cup] more complicated for [captain] Severin Luthi because he's trying to juggle these 2 players and make sure both of them are happy."
Rumours of discontent between the Swiss duo were heightened when Federer failed to fulfill his media duties on Sunday evening, conducting only one behind closed doors interview with an ATP journalist, and thereby avoiding awkward questions regarding a possible altercation with Wawrinka.
Speaking on his fitness for the tie against France, Federer said: "Probably in a few days it's going to be better, but right now it's not good enough. So clearly it's very disappointing.
"Recovery [is the plan] obviously as quick as possible, and then traveling to France at some point and getting ready on the clay for the Davis Cup final."
The stoking of relations between Switzerland's No.1 and No.2 ranked singles players threatens to derail preparations ahead of the pursuit of maiden Davis Cup crown title on the clay courts of Lille next week.
After a career which has included 17 grand slam titles, Olympic doubles gold and a record 237 consecutive weeks as world No.1, Davis Cup success is the only remaining title missing from Federer's CV.
However his relationship with Wawrinka – which has been key to Switzerland reaching a first final since 1992 - seemingly requires immediate surgery in order to prevent it jeopardising the team's chances.
Meanwhile, the ATP have confirmed that a part-refund will be offered to supporters at the O2 Arena, after many fans paid up to £110 for the final. The 19,000 capacity crowd were treated to Bob and Mike Bryan winning the doubles title in three sets after defeating Ivan Dodig and Marcelo Melo but many could have half of their ticket-price returned after the cancelled singles final.
ATP president Chris Kermode did however hastily filled the schedule with a first-to-eight pro-set between Andy Murray and Djokovic, which the British No.1 lost 8-5 but agreed to play for free despite his harrowing week in the English capital.
Fans were then treated to a legends doubles match as McEnroe and Murray teamed up against Tim Henman and Pat Cash, much to the delight of the supporters who stayed.