''I told him that he's the greatest of all time,'' Zverev said. ''I know that he was chasing history.'' Djokovic, the top-ranked Serb who had seemed invincible in all of the biggest tournaments this year, lost to Zverev 1-6, 6-3, 6-1 Friday in the semifinals of the tennis tournament at the Tokyo Olympics. 

Djokovic was attempting to become the first man to win all four Grand Slam tournaments and Olympic gold in the same year. He won the Australian Open, the French Open and Wimbledon this year and needed the Olympic and U.S. Open titles to complete the Golden Slam collection. 

''I feel so terrible right now,'' Djokovic said. ''I can't be positive right now.'' Steffi Graf in 1988 remains the only tennis player to achieve the Golden Slam. But Djokovic can still go after the calendar-year Grand Slam by winning the U.S. Open — something no man has accomplished since Rod Laver in 1969. 

''He won 20 Grand Slams,'' Zverev said. ''So you can't have everything. 

''At the end of the day he’s the greatest of all time, because he’s going to win the most Grand Slams, he’s going to win the most Master Series, he’s going to be the longest at the world No. 1, and I’m sure 99% that this is the case when it's all said and done,'' Zverev said. 

Zverev’s opponent in the gold-medal match will be Karen Khachanov. The Russian beat Pablo Carreno Busta of Spain 6-3, 6-3. 

Djokovic will play two bronze-medal matches on Saturday. He'll face Carreno Busta in singles then will team with Serbian partner Nina Stojanovic in mixed doubles against Ash Barty and John Peers of Australia. 

On a humid and muggy evening, Djokovic committed a series of uncharacteristic errors after a strong start as the 6-foot-6 (1.98-meter) Zverev started to win free points with his big serve. 

Djokovic was up a set and a break in the second before Zverev won 10 of the final 11 games. 

''It's just sport. He played better,'' Djokovic said. ''I got to give him credit for turning (the) match around. He served extremely well. I mean I was not getting too many looks on the second serve. My serve just drastically dropped. I didn’t get any free points from 3-2 up in the second. My game fell apart.'' Djokovic hadn’t lost since getting beaten by Rafael Nadal at the Italian Open final 2½ months ago. 

''He’s gone these last few months without any losses and for him it’s a big loss ... just collapsing in the end,'' said Marin Cilic of Croatia, who won silver in doubles. 

Cilic noted that the extreme heat and humidity at the Ariake Tennis Park throughout the tournament may have caught up with Djokovic. 

''It’s very unfortunate for him but still he’s an unbelievable champion and he still has so many years ahead of him to achieve many great things,'' Cilic said. 

When Zverev hit a backhand winner down the line that Djokovic didn’t move for to close it out, Djokovic walked to the net where he received a warm hug from Zverev as the pair exchanged some words. 

''Of course I’m happy that I’ve won, but in the end of the day I also know (how) he feels,” Zverev said. 

Djokovic’s only Olympic medal was bronze in singles at the 2008 Beijing Games — his first. 

Djokovic came back on the court little more than an hour later with Stojanovic in the mixed semifinals and lost that match, too, getting beaten by the Russian duo of Elena Vesnina and Aslan Karatsev 7-6 (4), 7-5. 

''I feel terrible right now in every sense but tomorrow hopefully fresh start I can recover and at least win one medal for my country,” Djokovic said. 

It was the third consecutive day that Djokovic played two matches. 

Viktor Troicki, Serbia's Olympic team coach, told The Associated Press on Thursday that the entire team was against Djokovic playing mixed doubles because they didn't want the event to tire him out with so much on the line in singles. 

''I’m sure he’ll get stronger after this and hopefully learn a few things,'' Troicki said Friday. 

The 34-year-old Djokovic is the first man since Laver that year to win the first three major tournaments in a season. 

He also matched his longtime rivals Roger Federer and Nadal by winning Wimbledon this month for his 20th Grand Slam title. He was the only member of the Big Three to travel to Tokyo and Djokovic has been soaking in the OIympic experience like few other athletes. 

Without Djokovic, though, the gold-medal match in singles will be lacking star power. Zverev’s best career result was reaching the final of last year’s U.S. Open, while the 25th-ranked Khachanov is coming off a quarterfinal appearance at Wimbledon and is now in the biggest final of his career. 

Zverev called it ''maybe the proudest moment of my career so far. 

''Because I'm not only playing for myself I’m not only playing for my parents, for my brother, for my family,” he added. “But I’m also playing for everybody, all the (German) athletes here back at the base and everybody back at home watching.'' In men's doubles, the top-seeded pair of Nikola Mektic and Mate Pavic beat Cilic and Ivan Dodig 6-4, 3-6, 10-6 in an all-Croatian final. It was Croatia's first gold and first silver in Olympic tennis, previously having won three bronzes. 

The New Zealand team of Marcus Daniell and Michael Venus took bronze by beating Austin Krajicek and Tennys Sandgren of the United States 7-6 (3), 6-2.