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 collection. 

Steffi Graf in 1988 remains the only tennis player to achieve the Golden Slam. 

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 Carreno Busta for bronze. 

Djokovic's hadn't lost since getting beaten by Rafael Nadal at the Italian Open final 2½ months ago. 

On a humid and muggy evening at the Ariake Tennis Park, 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. 

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 was embraced by Zverev. Djokovic responded by resting his head on Zverev's shoulder as the pair exchanged some words. 

Djokovic's only Olympic medal was bronze in singles at the 2008 Beijing Games — his first. He could still win a gold at the Tokyo Games in mixed doubles. 

Djokovic was due back on the court almost immediately to play with Serbian partner Nina Stojanovic in the mixed doubles semifinals against the Russian duo of Elena Vesnina and Aslan Karatsev.