Novak Djokovic was dramatically disqualified from the US Open on Sunday after accidentally striking a female line judge with a ball in frustration during his last-16 match, sending shock waves through the tournament.

The world number one apologised for hitting the ball in disgust after losing his serve to go 6-5 down in the first set to Spain's 20th seed Pablo Carreno Busta inside Arthur Ashe Stadium.

Djokovic appeared to be looking the other way when he pulled the ball from his pocket and smacked it in the direction of the official, the ball hitting her full toss in the throat.

The official cried out and began gasping for air as she collapsed to the ground.

Djokovic rushed over to check on her, placing his hand on her back as she struggled to breathe. After a few minutes she got up and walked off the court looking dazed.

Around ten minutes of discussions then ensued between Djokovic and the tournament referee Soeren Friemel, during which the world number one pleaded his case.

The umpire then declared that Carreno Busta had won by default. Djokovic shook hands with his opponent before leaving the court without appearing to shake the umpire's hand.

'So wrong'

He went then directly to his car and left the Flushing Meadows site in New York without speaking to reporters.

Djokovic, 33, later posted an apology on Instagram saying he was “so sorry.”

“This whole situation has left me really sad and empty,” he wrote, adding that “thank God” the woman was okay.

“I'm extremely sorry to have caused her such stress. So unintended. So wrong,” Djokovic said. He also apologised to tournament organisers for “my behavior” but did not say whether he thought they were right to default him.

The United States Tennis Association (USTA) said Djokovic had been defaulted under the Grand Slam rules for “intentionally hitting a ball dangerously or recklessly within the court or hitting a ball with negligent disregard of the consequences”.

The body added that he would lose all ranking points and prize money from the tournament.

Referee Friemel said Djokovic told him he should not be defaulted because it was unintentional.

Friemel said he agreed there was no intent but that it was a clear-cut case of Djokovic hitting the ball “angrily and recklessly”.

“She was clearly hurt and in pain. There was no other option,” he told reporters.

The Serbian star is one of only a handful of players to be disqualified from a men's singles tournament at a Grand Slam since John McEnroe was infamously tossed from the Australian Open in 1990.

Djokovic had been chasing an 18th Grand Slam title at the Billie Jean King US National Tennis Centre. He was hoping to close the gap on Roger Federer and Rafael Nadal, both absent from the tournament, in the race for the all-time men's Slam singles title record.

Djokovic is on 17, with Nadal on 19 and Federer on 20. Carreno Busta said he did not see the incident.

“I was celebrating the break with my coach,” he told reporters. “When I heard that the line judge was on the floor I was in shock. I never expected this moment when playing Novak.

“I think it was bad luck,” he added.

Former players reacted with shock at the disqualification but said officials had made the correct decision.

New winner

“Right call!” former women's world number one Tracy Austin wrote on Twitter.

Four-time US Open champion Martina Navratilova said officials “had no choice” but to default Djokovic.

Former British number one Greg Rusedski described the disqualification as “unbelievable” but said officials had made the “correct decision”.

A USTA spokesperson said the line judge was “doing okay” after receiving treatment from medical staff. Djokovic had been the overwhelming favorite to capture a fourth US Open title.

His disqualification blows the men's draw wide open and means it will crown a first-time Grand Slam winner next Sunday.

“Now it gets interesting,” said fifth seed Alexander Zverev, who described Djokovic as “very unlucky.” “It's going to be one of the young guys,” who wins, Zverev added after becoming the first German to reach the US Open last eight since 2007.

Fifth seed Zverev bulldozed Spain's unseeded Alejandro Davidovich Fokina 6-2, 6-2, 6-1 while 12th seed Canadian Denis Shapovalov beat David Goffin for a maiden Grand Slam last-eight spot.

In the women's draw, American Jennifer Brady, seeded 28th, stunned three-time Grand Slam winner Angelique Kerber while eighth seed Petra Martic was bundled out by Kazakhstan's 23rd seed Yulia Putintseva.

And American Shelby Rogers upset sixth seed Petra Kvitova 7-6 (7/5), 3-6, 7-6 (8/6). She will next play two-time Slam winner Naomi Osaka, who defeated Estonia's Anett Kontaveit 6-3, 6-4.