Fans have been slammed for disrespecting Novak Djokovic during the Serb's ATP Finals defeat to Roger Federer.
Federer produced a near-flawless performance to avenge his Wimbledon defeat by Djokovic, qualifying for the last four with a 6-4, 6-3 victory.
However, the Swiss maestro's epic performance was overshadowed somewhat by the behaviour of the crowd in London, with several former players expressing their outrage.
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As is normally the case when Federer plays, the 20-time grand slam champion had the majority of fans on his side.
But it was the loud cheers that accompanied Djokovic errors - double faults in particular - that had many viewers riled up.
The Serb was hindered by an elbow injury but still managed to play some superb tennis, although even then it was only met by subdued applause by fans inside the stadium.
Big-serving former men's star Greg Rusedski, commentating for Amazon Prime, slammed the reception for Djokovic.
“I think it’s disrespectful when you’re clapping double faults. That’s too much,” Rusedski said.
“I don’t mind fans supporting one guy over the other — that’s your choice — but there’s a certain etiquette you have to have out there.
“That really got into Djokovic’s head because he double faulted, some of the crowd applauded and he double faulted again. Then he got broken and from that point on he was always behind in the match.
Retired former women's World No.5 Daniela Hantuchova also called on fans to show Djokovic more respect.
“What else does he (Djokovic) need to do to get respect from this crowd? I think it got to his head a little bit. He didn’t even wave as he went off and that’s understandable. It’s not pleasant when someone claps your double faults so I can understand his frustration,” she said.
“After everything he’s done it’s a little bit disrespectful. Roger is Roger but at the same time Novak is Novak so maybe a more even crowd would have been ideal tonight.”
Djokovic looked nervy at the start of the winner-takes-all contest, double-faulting twice in the third game, in which he was broken to love.
Crowd spurs Federer on to victory
As cries of "Let's go Roger, let's go" rang around the cavernous stadium, Federer was dead-eyed on his serve, hitting eight aces, including a second-serve ace, in the first set.
Federer's service level dipped in the second set and 32-year-old Djokovic earned his first break point of the match in the fourth game, which the Swiss saved.
In the next game Djokovic slipped to 15-40 and sailed a forehand long to give Federer his second break of the match. The Swiss broke once more to close out victory.
Federer started the tournament with a chastening straight-sets defeat to Dominic Thiem but found his best form when it mattered.
Defeat for the second seed spells the end of his bid to overtake Rafael Nadal and finish as year-end number one.
Roared on by a raucous packed house at London's O2 Arena, six-time champion Federer looked in the groove from the start, cranking up the pressure on Djokovic's serve and dropping just three points on his own serve in the first set.
The Serbian upped his game at the start of the second set but Federer, 38, saved the one break point he faced and broke twice to canter to victory.
The third seed, making his 17th appearance at the ATP Finals, is into his 16th semi-final at the year-end event.