Alexander Zverev had no need to be sorry for stopping play and did not deserve to be booed, Roger Federer said after his ATP Finals last-four defeat.
Zverev halted play in their second-set tie-break after a ball boy came on to the court to retrieve a dropped ball.
The point was replayed according to the rules but that did not stop the jeers.
Interviewer Annabel Croft told fans to be "more respectful" during Zverev's victory speech and Federer hoped the boy did not "have a sleepless night".
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Zverev, who should have been celebrating the 7-5 7-6 (7-5) victory that put him into Sunday's final of the season-ending tournament at London's O2 Arena, was instead taking time to compose himself in his on-court interview and apologising to Federer and the crowd.
"I was really upset afterwards in the locker room, I'm not going to lie. I had to take a few minutes for myself," Zverev, 21, said after one of the biggest wins of his career.
"But, you know, I hope the crowd and the people who were booing maybe look at what actually happened, maybe just realise that I've maybe not done anything wrong."
Croft came to his defence, sharply telling the crowd: "I'm not sure why you are booing; he is telling the truth. The ball boy moved across the court and disrupted play.
"I think you need to be a little bit more respectful; they are the rules."
Federer said the whole episode had been "unfortunate".
He said: "Sascha [Zverev] doesn't deserve it. He apologised to me at the net. I was like, 'Buddy, shut up. You don't need to apologise to me here.'
"He shouldn't be apologising. 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."
It was 3-4 in the second-set tie-break on Zverev's serve when the German stopped a baseline rally when the ball boy fumbled a ball at the far end of the court.
Initially Federer looked perplexed by what was happening but umpire Carlos Bernardes checked with the ball boy and the point was replayed.
The partisan Federer crowd were unhappy, even more so when Zverev delivered an ace to level at 4-4 and went on to win the tie-break and the match.
"It's a bold move by Sascha to stop the rally because the umpire can just say: 'Sorry, buddy, you're in the rally. I don't care. You lost the point. I didn't see it.'
"That's where I just wanted to double-check with the umpire, what is the situation. But not for a second was there a sportsmanship situation there.
"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."
Federer, who was seeking a seventh title here, said he was not cross with the ball boy.
"It's OK, no problem, that happens," the Swiss said. "It's all good. It's all good. I hope he doesn't have a sleepless night. It's not a big deal at the end of the day.
"I'm definitely not mad at him."