Novak Djokovic's hopes of becoming the first man to win a calendar Golden Slam were crushed by a semi-final defeat to Alexander Zverev at the Tokyo Olympics.

From a set and a break up, Djokovic dropped a staggering eight games in a row on his way to a 1-6 6-3 6-1 loss.

The 34-year-old Serbian had been unsure about coming to the Games but was swayed by the pride he takes in representing his country and the tantalising opportunity to add a gold medal to a potential clean sweep of the grand slams.

He has already won the Australian Open, French Open and Wimbledon, and will head to the US Open in August as a heavy favourite regardless of this setback.

But this was just not Djokovic's day, despite him making a whirlwind start and dominating until the point he broke to lead 3-2 in the second set. From there, Zverev seized control. 

When Djokovic volleyed into the net to make it 3-3, dropping serve for the first time in the match, it looked like just a minor stumble.

Yet suddenly he was struggling for form and did not win another game until he was already 4-0 behind in the deciding set.

Zverev was playing blindingly brilliant tennis and was proving obdurate too, saving four break points in the second game of the third set.

He clinched victory with a blazing backhand winner, and goes on to face Russian Karen Khachanov in the final.

The result means Djokovic, like Roger Federer, seems fated never to win the Olympic singles gold medal. And it leaves Steffi Graf as the only player to ever win a calendar Golden Slam, having done so in 1988 when she added the Seoul Olympics title to her haul of majors.

Russian Olympic Committee's Khachanov fended off Spain's Pablo Carreno Busta with some ease in the first semi-final, with the world number 25 sweeping to a 6-3 6-3 victory in an hour and 19 minutes.

He won 26 of 28 points on first serve to freeze out Carreno Busta, who had only one break point all match and could not take that opportunity.

"It's just a pure happiness, a pleasure to be here to live those moments, these kind of memories will stay forever," Khachanov said.

Khachanov delivered a rock solid display, with his serve and forehand at their best, barely giving his opponent a sniff of an opportunity as the Moscow-born 25-year-old established a firm grip.

"That's the way I prepared, against every opponent you play a little bit differently," Khachanov said. "The final will be another story, another match, another day. I hope it will be the same."

Carreno Busta, who will face Djokovic for the bronze medal, said: "It was not the best match I have played, but Karen was unbelievable today, playing very aggressive and serving really good."