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 Annabelle Croft told fans to be "more respectful" during Zverev's victory speech and Federer hoped the boy did not "have a sleepless night".
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.