Novak Djokovic was kicked out of the U.S. 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 Carreño 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 tournament referee Soeren Friemel, Grand Slam supervisor Andreas Egli and chair umpire Aurelie Tourte, Djokovic pleaded his case.

“His point was that he didn't hit the line umpire intentionally. He said, ‘Yes, I was angry. I hit the ball. I hit the line umpire. The facts are very clear. But it wasn't my intent. I didn't do it on purpose.’ So he said he shouldn't be defaulted for it,” said Friemel, who made the decision to end the match. “And we all agree that he didn't do it on purpose, but the facts are still that he hit the line umpire and the line umpire was clearly hurt.”

Friemel didn't see what happened, and said he was not allowed to check a video replay, but was given a rundown by Egli and Tourte. Friemel said that even if Djokovic didn't intend to hurt the line judge, she was hurt, and that was enough to merit the ruling.

Eventually, Djokovic walked over to shake hands with Carreño Busta. Tourte then announced that Djokovic was defaulted, the tennis equivalent of an ejection.

“I was a little bit in shock, no?” Carreño Busta said later at a news conference done via video conference because of social-distancing rules at the U.S. Open, the first Grand Slam tournament staged amid the coronavirus pandemic.

Djokovic quickly left the tournament grounds without speaking to reporters, posting an apology on social media hours later.

“This whole situation has left me really sad and empty. I checked on the lines person and the tournament told me that thank God she is feeling ok. I‘m extremely sorry to have caused her such stress. So unintended. So wrong,” Djokovic wrote.

“As for the disqualification, I need to go back within and work on my disappointment and turn this all into a lesson for my growth and evolution as a player and human being,” he wrote. “I apologize to the @usopen tournament and everyone associated for my behavior.”

Asked whether he thought Djokovic should have been allowed to continue to play, Carreño 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 U.S. 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 $250,000 in prize money he earned in the tournament — “in addition to any or all fines levied with respect to the offending incident.”

“Novak was angry. He hit the ball recklessly, angrily back. And taking everything into consideration, there was no discretion involved," Friemel said. “Defaulting a player at a Grand Slam is a very important, very tough decision. And for that reason, it doesn't matter if it’s on Ashe, if it’s No. 1, or any other player on any other court, you need to get it right.”

This was the latest example of Djokovic finding himself at the center of the tennis world for a reason other than his best-in-the-game returns, can’t-miss groundstrokes and body-contorting defensive prowess.

Djokovic tested positive for the coronavirus — as did his wife, one of his coaches and other players — after participating in a series of exhibition matches with zero social distancing he organized in Serbia and Croatia in June.

Then, on the eve of the U.S. Open, he helped establish a new association he says will represent men's tennis players.

And, of course, there's been his dominance on the court.

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 U.S. Open champion Nadal, who cited concerns about traveling amid the pandemic, and Federer, sidelined after two knee operations, not in the field, the 33-year-old from Serbia was expected to claim a fourth trophy in New York and gain on them.

But it all came apart so suddenly Sunday. Djokovic wasn't looking in the line judge's direction when his racket made contact with the ball, and there was concern on his face as soon as he realized what had happened.

Players who hit a ball out of anger and make contact with an on-court official have been defaulted in the past.

In 2017, 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.

“Very unlucky for Novak,” said No. 5 seed Alexander Zverev, who will face No. 27 Borna Coric in the quarterfinals. “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 U.S. Open, which has no spectators, is that only the two largest arenas — Ashe and Louis Armstrong Stadium — have full complements of line judges. Elsewhere, 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, then whacked a ball off a courtside advertising sign.

On the second point of what would become Djokovic's last game at this year’s U.S. Open, he stumbled and fell, clutching his left shoulder.

Play was delayed for a few minutes while a trainer checked on him.

On the second point after they resumed, the 20th-seeded Carreño 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 U.S. 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.”