Novak Djokovic was kicked out of the US Open for accidentally hitting a line judge in the throat with a tennis ball after dropping a game in his fourth-round match Sunday, a stunning end to his 29-match winning streak and bid for an 18th Grand Slam title.

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 favourite 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. But eventually, he walked over to shake hands with Carreno Busta.

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

"I was a little bit in shock, no?" Carreno Busta said later at a news conference done via video conference.

Djokovic left the tournament grounds without speaking to the media.

Asked whether Djokovic should have been allowed to continue to play, Carreno Busta shrugged and replied: "Well, the rules are the rules.... The referee and the supervisor (did) the right thing, but it's not easy to do it."

Indeed, the US Tennis Association issued a statement saying that Friemel defaulted Djokovic "in accordance with the Grand Slam rulebook, following his actions of intentionally hitting a ball dangerously or recklessly within the court or hitting a ball with negligent disregard of the consequences".

The USTA went on to say Djokovic forfeits the ranking points and USD 250,000 in prize money he earned in the tournament -- "in addition to any or all fines levied with respect to the offending incident".

Djokovic began the day 26-0 this season and with an unbeaten run that extended to his last three matches of 2019. He had won five of the past seven Grand Slam tournaments to raise his total to 17, closing in on rivals Roger Federer, who has a men's-record 20, and Rafael Nadal, who has 19.

With reigning US Open champion Nadal, who cited concerns about travelling amid the pandemic, and Federer, sidelined after two knee operations, not in the field, the 33-year-old Djokovic was expected to claim a fourth trophy in New York and gain on them.

But it all came apart so suddenly Sunday, even if it was clear that Djokovic did not intend to hit the line judge. He wasn't looking in her direction when his racket made contact with the ball, and 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.

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.

"I think the supervisors and all them are just doing their job, but very unlucky for Novak," said Alexander Zverev, the 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 US Open, the first Grand Slam tournament since the outset of the 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.

Then, on the second point of what would become the last game at this year's U.S. 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 on him.

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 left in the field who 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.

Plus, each of the last 13 Grand Slam trophies had been won by a member of the Big Three of Federer, Nadal and Djokovic.

"Going to be a new Grand Slam champion, (that's) all I know. No Grand Slam champions left in the draw. Now it gets interesting," said Zverev, who beat Alejandro Davidovich Fokina 6-2, 6-2, 6-1. "Now I think is the time when it gets really interesting."