NEW YORK (AP) — Top-seeded Novak Djokovic has been defaulted from his fourth-round match at the U.S. Open after he accidentally hit a line judge with a tennis ball.

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.

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