Top-seeded Novak Djokovic was defaulted from his fourth-round match on Sunday at the US Open after a tennis ball he hit in anger struck a line judge in the neck.

As he walked to the Arthur Ashe Stadium sideline for a changeover, trailing Pablo Carreno Busta 6-5 in the first set, Djokovic — who was seeded and ranked No. 1 and an overwhelming favorite for the championship — angrily smacked a ball behind him. The ball flew right at the line judge, who dropped to her knees at the back of the court and reached for her neck.

During a discussion of about 10 minutes near the net involving chair umpire Aurelie Tourte, tournament referee Soeren Friemel and Grand Slam supervisor Andreas Egli, Djokovic appeared to be pleading his case. Eventually, he walked over to shake hands with Carreno Busta.

Tourte then announced that Djokovic was defaulted, the tennis equivalent of an ejection.

Djokovic left the tournament grounds without speaking to the media. In an Instagram post, he apologized to the line judge, saying he didn’t intend to hit her and that he is going to “turn all this into a lesson for my growth and evolution as a player and human being.”