Roger Federer felt fans who booed Alexander Zverev at the O2 Arena were out of order after the German secured his place in the ATP Finals decider and denied him a 100th singles title.
Zverev stopped mid-rally during a second-set tie-break when a ball boy in his eyeline dropped a ball with Federer leading 4-3.
A Zverev ace flashed past Federer when the point was replayed and the Swiss soon bowed out with a 7-5 7-6 (7-5) defeat in London.
The crowd voiced their disapproval after Zverev halted the point and he apologised after he was also booed and jeered during his on-court interview, but Federer said the third seed did not need to apologise.
Federer said: "He apologised to me at the net. I was like, 'buddy, shut up! You don't need to apologise to me here. Congratulations on a great match and a great tournament so far.'
"He didn't do anything about it. He just called it how it was. He felt it affected play. There is a rule that if something like this happens, obviously you replay points."
The 20-time grand slam champion added: "Look, I understand the frustration. It's just unfortunate circumstances,
"Booing, I never like it. We see it in other sports all the time, but in tennis it's rare.
"So when it happens, it gets very personal and we take it very direct. I think it's unfortunate that this happened. Sascha doesn't deserve it."
Federer said his season will go down as a success despite a disappointing finish.
"I must tell you I'm very proud that at 37 I'm still so competitive and so happy playing tennis." said the Swiss.
"From that standpoint, as disappointed as I might be about this match, if I take a step back, I'm actually very happy about the season,
"It's been a historic season in some ways. Got back to world number one. For me, that was a huge moment in my life, to be honest, in my career because I never thought I would get there again."
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