What is a Golden Slam? Novak Djokovic's path to history stopped by Zverev

Only one player, Steffi Graf, has pulled it off.

On the warpath to an extremely rare Golden Slam after a dominant two years, Djokovic lost to Germany's Alexander Zverev in the Olympic semifinal in Tokyo on Friday 1-6, 6-3, 6-1.

"He won 20 Grand Slams," Zverev said after the match, according to the Associated Press, referring to the 20 major titles the Serbian player has won. "So you can't have everything."

Djokovic will have the opportunity to play for the bronze medal. He previously won bronze in 2008.

He is also competing in the mixed doubles at the Olympics alongside Nina Stojanovic, in what was a controversial decision as his team didn't want him to risk injury or fatigue, per AP. After also losing with her on Friday, he'll be playing for another bronze there.

Every year, tennis players have the opportunity to achieve a Grand Slam by winning all four majors: the Australian Open, French Open, Wimbledon and U.S. Open.

But every four years -- or, well, five years, in this case -- they have the opportunity to achieve a "Golden Slam." That means winning all four majors and the Olympics.

It's only ever been achieved once, by Steffi Graf in 1988. No man has ever completed a Golden Slam.

Djokovic, at the very least, was close to it. So far this year, he had won the Australian Open, French Open and Wimbledon. The U.S. Open takes place at the end of summer.

A Grand Slam is rare enough. No men's tennis player has done it in a calendar year since Rod Laver in 1969 -- though Djokovic did hold all four titles simultaneously from 2015 to 2016.

He faced a somewhat limited field in Tokyo as Rafael Nadal, Roger Federer and Matteo Berrettini, who Djokovic beat in the Wimbledon finals to tie Nadal and Federer's majors record, will not be competing.

Despite that -- and despite the Serbian athlete's general dominance over the last two years -- it wasn't exactly an easy field as Djokovic's competition in the tournament included Greece's Stefanos Tsitsipas, Zverev and Russia's Daniil Medvedev, representing the Russian Olympic Committee, among others.

Djokovic made no secret of his ambitions, telling Montenegro's MINA news agency before the Games, according to USOpen.org, "I am inspired to play my best tennis and confident that I can win the gold medal after a tremendous run so far this season."

While he acknowledged the Olympics and U.S. Open would be "demanding," he added, "But I am full of confidence and motivated to represent Serbia in the best possible way. I am yearning for a medal in Tokyo, hopefully gold, and then I'll go to New York aiming to complete it all."

For more Olympics coverage, see: https://abcnews.go.com/Sports/Olympics

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