'Are you kidding': Disbelief over 'insane' US Open scenes

Novak Djokovic and Alexander Zverev have left the tennis world scrambling for superlatives after an extraordinary rally in their US Open semi-final in New York.

World No.1 Djokovic was taking on Zverev for a place in the final at Flushing Meadows, hoping to move a giant step closer to a record-breaking 21st major that would also complete a rare calendar year Grand Slam.

'WAIT FOR IT': Tennis fans react to Djokovic drama at US Open

'WOW': Daniil Medvedev's 'brutal' US Open act stuns tennis world

'ABSURD': Insane history made as teens storm into US Open final

No male player has won all four majors in the same year since Australia's Rod Laver in 1969.

For the fourth straight match at the US Open, the 20-time grand slam champion dropped the opening set, with Zverev coming out of the blocks flying.

There was a sense of inevitability as Djokovic stepped up a gear by bouncing back from the setback in the opening stanza to claim the next set.

In the third set, nothing could seperate the players until the tenth game when Djokovic set up a triple break point chance.

After saving the first, Zverev was forced to dig deeper than anyone has at this year's US Open to save another against the World No.1.

Both men engaged in the longest exchange at this year's tournament - an extraordinary 53-shot rally that left viewers in disbelief.

When the dust settled on the remarkable rally it was Zverev soaking in the applause from a raucous crowd inside Arthur Ashe stadium that had risen to its feet to salute a truly extraordinary point.

However, Djokovic kept the relentless pressure going and finally converted his third break point chance to take an ominous two-sets-to-one lead in scenes that left the tennis world in a frenzy on social media.

Novak Djokovic powers past gutsy Alexander Zverev

Djokovic's stunning display would have broken the spirit of many opponents but to his credit, Zverev dug deep in the next set to fight his way back.

The German consolidated an early break to jump out to a 5-3 lead and force Djokovic to hold serve to stay in the set.

The World No.1 managed to do just that but he had no answer to Zverev's big-hitting in the fourth as the 24-year-old sent the match into a deciding fifth set.

Djokovic fired the first major salvo in the decider after winning another scintillating rally to break Zverev's opening service game.

The Serb was relatively untroubled on his own racquet as he raced out to a 3-0 lead to put the pressure firmly on his opponent.

The cracks began to open for Zverev in the fourth game as two wayward shots gave Djokovic a crucial double break, which he consolidated on his own serve to move just one game away from a place in the final.

Zverev finally stemmed the bleeding to hold serve and make the Serb serve it out for the win.

The German refused to go down without a fight though and set up two break point chances to give himself the faintest sniff of hope.

Time and time again, Djokovic managed to come up with something big when he needed it most but a rare double fault eventually handed Zverev a break back of his own.

The lengthy rallies continued in the eighth game but when Zverev sent a forehand long to give Djokovic another break point, the World No.1 capitalised to book a date with Daniil Medvedev in the final.

with AAP

Click here to sign up to our newsletter for all the latest and breaking stories from Australia and around the world.