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Djokovic was becoming frustrated after failing to take the opening set and having dropped serve to trail 5-6, he hit a ball towards the back of the court, striking a lineswoman.
He immediately apologised, before becoming involved in a lengthy exchange with the tournament referee.
Under the rules of tennis, there was little option other than a default and despite Djokovic's pleadings that he had not meant to hit the official, he was eventually disqualified.
It was an incredible end to the top seed's hopes of winning an 18th grand slam singles title - something that had looked increasingly likely as the fortnight progressed.
"It's the right decision," Tim Henman, who was disqualified from Wimbledon for a similar incident in 1995, 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."
With Djokovic out of the US Open, a first-time men's singles grand slam champion is set to be crowed at Flushing Meadows - LIVE on SBS from 6am Monday (AEST).