Disqualification worst moment of Djokovic's career, says Becker

'He did break the rule, the decision is correct.'

Novak Djokovic's stunning disqualification from the US Open on Sunday for striking a line judge with a ball is the most difficult moment in his career, said his former coach Boris Becker.

World number one Djokovic was defaulted at 5-6 down in the first set of his match against Pablo Carreno Busta after a ball he swiped in frustration struck a female official in the throat, leaving her in distress.

"I'm as shocked as anybody. Novak and me go way back. We call each other family," Becker, working as an analyst for broadcaster Eurosport, said. "This is probably the most difficult moment in his entire professional life. He did break the rule, the decision is correct."

Djokovic, a huge favourite to win his fourth US Open title, was becoming exasperated after failing to convert three set points and then falling heavily and requiring treatment on his shoulder.

The 33-year-old, bidding to claim an 18th Grand Slam title and move within two of Roger Federer's men's record, then had his serve broken at 5-5 by the Spaniard.

"Thankfully there were no people, no spectators, but he has a habit sometimes of hitting the ball in the stands. Frustrations started boiling up," Becker said.

"The ball was hit in the lineswoman's throat, she fell backwards and she was looking for air. This wasn't intention, he didn't want to hit the line judge, but he did hit the ball and those are the rules. Tough one."

Djokovic spent 12 minutes pleading his case but eventually walked off looking stunned at what had happened.

"Maybe leaving the court at the end was the toughest moment in Novak Djokovic's career," Becker, a former US Open champion, said. "He was in the form of his life, undefeated this year, on his way to collecting Grand Slam number 18. None of that, now he's going to go home.

"It's only the middle Sunday and we already know we will have a new men's winner -- what a crazy tournament."