Video footage has emerged that appears to show Roger Federer's wife sledging her husband's opponent by calling him "a cry baby" during a tense semi-final match.
Federer beat fellow Swiss Stan Wawrinka at the ATP Tour Finals in three sets 4-6, 7-5, 7-6 on Saturday night at the 02 Arena.
Wawrinka became agitated towards the end of the match and footage reveals a female voice - which appears to belong to Mirka Federer, a former professional player and mother of her husband's four children, call him a "cry baby" when he takes issue with her shouting encouragement to her husband just before he serves.
Wawrinka moves towards her asking: "What did she say?" The umpire asks him to continue playing and the crowd begins booing.
Mirka Federer was watching the game from a ground-level supporters' box, from which comments and heckles are audible to the players.
Tensions were so high that Federer and Wawrinka began arguing in the tunnel after Federer won the match, and were made to "sort out their differences", The Telegraph reported. They reportedly argued for 10 minutes.
Despite his victory, Federer pulled out of the final, handing the title to Novak Djokovic by default.
"Unfortunately I'm not match fit," the 33-year-old told the crowd at the O2 Arena. "I tried everything I could last night, also today: painkillers, treatment, rest, so forth, warm-up, until the very end.
"But I just can't compete at this level with Novak. It would be too risky at my age to do this right now and I hope you understand."
He later tried to play down tensions between himself by tweeting an image of them together with their Swiss teammates ahead of the Davis Cup final.
The Davis Cup final is this Friday, when Federer and Wawrinka are due to play against France.
The night of his match against Federer, Wawrinka was asked what had made him upset on the court.
He said: “Not much. Nothing special. Tense match. It's never easy.”