By Amy Tennery

NEW YORK (Reuters) - U.S. Open Tournament Referee Soeren Friemel said there was "no other option" but to disqualify world number one Novak Djokovic on Sunday after he struck a line judge with a ball.

A frustrated Djokovic swatted a ball to the back of the court inside Arthur Ashe Stadium, inadvertently hitting a line judge in her throat after his fourth-round opponent broke his serve in the first set on Sunday.

Friemel told reporters at Flushing Meadows that the action warranted Djokovic's removal from the tournament, "based on the fact that the ball was hit angrily, recklessly, that it went straight at the line umpire's throat," even though he believed the 33-year-old did not intend harm.

"There are two factors, one is the action and the result," he said. "And the action – while there was no intent – the result of hitting a line umpire and (her) clearly being hurt is the essential factor in the decision-making process here."

Friemel, who became tournament referee prior to the 2019 U.S. Open, said he had never defaulted a player in Grand Slam competition before, calling Sunday's saga "a very unique experience."

Djokovic, who was gunning for an 18th Grand Slam title at this year's event, apologized to the tournament in a social media post and said he was "extremely sorry" to the line judge. [nL8N2G30SI]

"Defaulting a player at a grand slam is a very important, very tough decision," Friemel said. "It doesn't matter if it's on Ashe, if it's number one or any other player on any other court - you need to get it right, you need to make the right decision.

"And everyone on court was aware of what's at stake."

(Reporting by Amy Tennery; editing by Richard Pullin)