Novak Djokovic has long divided opinion over his outspoken nature and petulant tendencies, but the 33-year-old Serb crossed the line on Sunday as he became the first world number 1 disqualified from a Grand Slam singles tournament after he accidentally hit a ball at a line judge in a flash of temper.

The shocking incident happened when he lost his serve to go 6-5 down in the first set to Spain's Pablo Carreno Busta during the fourth round of the U.S. Open. He showed his frustration by taking a ball out of his pocket and smacking it behind him, hitting a female line judge in the throat.

Djokovic clearly did not aim for the referee, but when he saw her collapse to the ground, he rushed to check on her. He attempted to console her by putting his hand on her back, while chair umpire Aurelie Tourte and tournament referee Soeren Friemel also joined him on the court.

Djokovic then had a heated conversation with Friemel before being thrown out of the tournament. "She doesn't have to go to the hospital for this," said Djokovic who pleaded for leniency. "You're going to choose a default in this situation? My career, Grand Slam, center stage?"

As Friemel stood firm in his decision, Djokovic quickly left the stadium, leaving Carreno Busta stunned.

"When I heard that the line judge was on the floor I was in shock. I never expected this moment when playing Novak," said Busta. "I think it was bad luck."

A late apology

Djokovic didn't attend the mandatory press conference to tell his side of the story, but hours after his exit, he took to Instagram to issue a public apology.

"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," he said.

"I'm extremely sorry to have caused her such stress. So unintended. So wrong. I'm not disclosing her name to respect her privacy. 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 added.

"I apologize to the U.S. Open tournament and everyone associated for my behavior. I'm very grateful to my team and family for being my rock support, and my fans for always being there with me. Thank you and I'm so sorry."

A costly mistake

Late on Sunday, The U.S. Tennis Association explained the reasons behind Djokovic's disqualification and confirmed that the top seed will lose all ranking points earned at the tournament and fined 250,000 U.S. dollars in prize money.

"In accordance with the Grand Slam rule book, 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 U.S. Open tournament referee defaulted Novak Djokovic from the 2020 U.S. Open," read a statement.

"Because he was defaulted, Djokovic will lose all ranking points earned at the U.S. Open and will be fined the prize money won at the tournament in addition to any or all fines levied with respect to the offending incident," it added.

Friemel told reporters that he agreed Djokovic had no intention of hurting the referee but that it was a clear-cut case of the Serb hitting the ball "angrily and recklessly." "She was clearly hurt and in pain. There was no other option," he said.

A right call

The officials' decision to disqualify the 17-time Grand Slam winner was supported by several former players. 

"I hope the line judge is okay. The rule is the rule. It is unfortunate for everyone involved, but in this specific situation the default was the right call," American tennis great Billie Jean King wrote on Twitter.

Two-time U.S. Open champion Tracy Austin agreed. "Right call! Stunning to watch this unfold. Hope lines person OK," he said.  

Former British number one Greg Rusedski said, "Unbelievable that Djokovic has been defaulted. This was the correct decision under the rules regardless of intent or not."

"Novak should have stayed for the press conference & apologize. We must take responsibility for our actions no matter how difficult the situation is," he added.

The view was echoed by Tim Henman, who was defaulted from Wimbledon for a similar incident in 1995 when he accidentally hit a ballgirl.

"It's the right decision. You have to be responsible for your actions, when you look at the facts you have to be defaulted," he said. "We wouldn't want to see the best player in the world go out of the tournament in these circumstances but you have to be responsible for your actions on the court."

A new king in waiting

Djokovic was the favorite to win the men's title at the U.S. Open and his disqualification means a new Grand Slam champion will emerge for the first time in six years.

"Now it gets interesting," said Alexander Zverev, who became the first German to reach the U.S. Open quarter-finals in 13 years after cruising past Alejandro Davidovich Fokina on Sunday.

"There's going to be a new Grand Slam champion. That's all I know right now," observed the world number seven. "There's no past Grand Slam champions left in the draw. It's going to be one of the young guys [who wins]."