NEW YORK -- Top-seeded Novak Djokovic was defaulted from his fourth-round match at the US Open after he accidentally hit a line judge with a tennis ball Sunday.

It was a stunning end to Djokovic's bid for an 18th Grand Slam title and his 26-0 start to this season.

Djokovic had just lost a game to his opponent, Pablo Carreno Busta, to fall behind 6-5 in the first set. As he walked to the sideline for the changeover, Djokovic smacked a ball behind him. The ball hit a female line judge, who dropped to her knees at the back of the court and reached for her neck.

After a discussion of several minutes with officials on court, including tournament referee Soeren Friemel, Djokovic walked over to shake hands with Carreno Busta. Chair umpire Aurelie Tourte then announced the default.

It seemed clear Djokovic did not intend to hit the line judge; there was concern written on his face as soon as he realized what happened. But players who hit a ball out of anger and make contact with an on-court official have been defaulted in the past.

Djokovic's mood had soured over the preceding few minutes. 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. Then, on the second point of what would become the last game at this year's US Open for Djokovic, he stumbled while chasing a shot and fell to the ground, clutching his left shoulder.

Play was delayed for a few minutes while a trainer checked that shoulder. On the second point after they resumed, Carreno Busta hit a passing winner to break Djokovic's serve. That's when Djokovic got himself into trouble.

His departure means there is no man remaining in the field who previously has won a Grand Slam singles title. Whoever emerges as champion will be the first first-time major trophy winner in men's tennis since 2014, when Marin Cilic won the US Open.