'This is nuts': Tennis world in disbelief at 'insane' US Open drama

The tennis world was left stunned on Wednesday as Novak Djokovic and Matteo Berrettini slugged it out in a marathon first set at the US Open.

Djokovic met Berrettini in a rematch of their titanic clash in the Wimbledon final with a place in the semi-finals on the line in New York.

'NEVER SEEN THAT': Tennis fans stunned by 'absurd' US Open moment

'DISGUSTING': Piers Morgan savaged over Emma Raducanu comment

And the two big hitters didn't disappoint as Djokovic and Berrettini went toe-to-toe in a dramatic first set.

The first game of the match went for over 10 minutes before Djokovic eventually held his serve.

The match then went past the hour mark with only nine games played and Djokovic holding a 5-4 lead.

However Berrettini grabbed the pivotal break to take a 6-5 lead before serving out the first set in a staggering one hour, 17 minute epic.

It marked the third-consecutive match that Djokovic had dropped the first set.

Fans flocked to social media in disbelief over the crazy slugfest.

Novak Djokovic sets up clash with Alexander Zverev

Unfortunately for Berrettini he couldn't maintain the level of play as Djokovic stormed into the semi-finals.

The World No.1 showed why he's the best in the business at grand slam level, advancing to a final four showdown with Alexander Zverev after a 5-7, 6-2, 6-2, 6-3 win.

The Serbian star is now just two wins away from completing a calendar year Grand Slam.

The World No.1 can join Rod Laver as the only male players to achieve the feat, with Aussie great Laver doing so for the second time in 1969.

Djokovic would be the first player ever to achieve the fabled feat on three different surfaces.

Earlier, Zverev extended his winning streak to 16 matches by easing past South African Lloyd Harris 7-6 (8-6) 6-3 6-4.

The German looms increasingly large as a potential roadblock to Djokovic's bid for the Grand Slam following his comfortable triumph on Wednesday at Flushing Meadows.

The victory leaves Zverev one win away from returning to a second consecutive US Open final and another shot at a first career major title after losing to Dominic Thiem last year.

Harris stepped out onto Arthur Ashe stadium court oozing energy, looking as if he had performed on tennis's biggest stage many times before.

The Cape Town player had good reason to be confident, having knocked out three seeds, Olympic silver medallist Karen Khachanov (25), Denis Shapovalov (7) and Reilly Opelka (22) on his way to his first career slam quarter-final.

But Zverev, who came into the contest on a career-best winning streak and having dropped one set en route to his quarter-final, was just too strong.

After trading breaks, Harris had his best chance in the opening set tie-break when he served on set point at 6-5 but could not convert.

But Zverev would not squander his opportunity, taking the next two points and the set, leaving a furious Harris smashing drink bottles to the ground during the changeover, spraying water all over the court and bringing out ball boys and girls for a clean-up.

Scenting blood, Zverev broke Harris at the first opportunity in the second set and that was all he would need to take a 2-0 lead.

Harris had rallied from two sets to one down against Olympic silver medallist Khachanov in the opening round but there would be no way back against the Tokyo Games gold medal winner, who went up a double break in the third before finishing off his opponent with a thundering ace.

with AAP

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