'Unbelievable what just happened on the court'

Reaction to Novak Djokovic's disqualification from the US Open for striking a line judge with a ball during his match against Pablo Carreno Busta on Sunday.

Novak Djokovic

"This whole situation has left me really sad and empty. I checked on the lines person ... thank God she is feeling ok. I'm extremely sorry to have caused her such stress. So unintended. So wrong," he wrote in an Instagram post.

"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. I apologize to the @usopen tournament and everyone associated for my behavior."

Carreno Busta

"I think this was not intentional," the Spaniard told reporters. "I don't think that any one of us, the players, do this (intentionally).

"It's just I broke his serve and he threw the ball. I think that it was bad luck, no? You cannot do this, but of course I think that Novak never, never wants to hit the line umpire.

"I'm so sorry about that, because it's not the way that I want to be in (the) quarter-finals."

Alexander Zverev, fifth seed

"It's very unfortunate that he hit the line judge, and especially where it hit her," Zverev told reporters.

"Very unlucky for Novak. I think he's going to be a little bit upset about it. If he would have hit it anywhere else, if it would have landed anywhere else, we are talking about a few inches, he would have been fine.

"I'm a little bit in shock right now."

Martina Navratilova, 18-times Grand Slam singles champion

"Unbelievable what just happened on the court," she wrote on Twitter. "Novak Djokovic defaulted for inadvertently but stupidly hitting a lineswoman in the throat with a ball and the officials had no choice but to default.

"Wow... #sad Glad the woman is ok- we must do better than that."

Billie Jean King, 12-times Grand Slam singles champion

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

Tim Henman, former player

"It's the right decision," Tim Henman told Amazon Prime. "He is not aiming for the line judge, but has hit the ball away and you have to be responsible for your actions."

Mats Wilander, seven-times Grand Slam singles champion

"You are not allowed to do that," he said. "It's as much bad luck as you can have on a tennis court. He didn't just roll the ball back to the ball kid, that's the bottom line.

"He hit it harder than he intended to, obviously a complete accident. It was a sign of frustration, yes. A little bit. But it doesn't matter, you are not allowed to do it."

Alex Corretja, former French Open finalist

"It's amazing how one centimetre can change not only the match, but the future of our sport, the history of our sport," he told Eurosport.

"With the rule as it is, you need to disqualify him. It's obvious they had no other choice, it's a pity..."

Tim Mayotte, coach and former player

"Almost every high-performance player I train does what Novak did in firing the ball," he posted on Twitter. "I tell them to stop it, and break the habit for this exact reason, they eventually hit someone and it won't be good. Yes, Novak deserved it."

Nick Kyrgios, Australian tennis player

"Swap me for jokers incident. ‘Accidentally hitting the ball kid in the throat’ how many years would I be banned for? 5? 10? 20?" he wrote on Twitter.