In 2017, for example, Denis Shapovalov — the 21-year-old Canadian scheduled to play his fourth-round match Sunday night — was defaulted from a Davis Cup match against Britain when he accidentally hit the chair umpire in the face with a ball.

At Wimbledon in 1995, Tim Henman hit a ball into the head of a ball girl and was defaulted from a doubles match with partner Jeremy Bates.

“I think the supervisors and all them are just doing their job, but very unlucky for Novak,” said Alexander Zverev, the tournament’s No. 5-seeded man, who reached the quarterfinals by winning Sunday. “If it would have landed anywhere else — we’re talking a few inches — he would have been fine.”

Among the many oddities about the 2020 U.S. Open, the first Grand Slam tournament since the outset of the coronavirus pandemic, is that there are no spectators.

Another is that only the two largest arenas — Ashe and Louis Armstrong Stadium — have full complements of line judges making calls at matches. At other courts, chair umpires are aided by an electronic line-calling system.

Djokovic’s mood had soured over the preceding few minutes Sunday. In the prior game, he wasted three consecutive break points and after the last, which Carreno Busta won with a drop shot, Djokovic whacked a ball off a courtside advertising sign.