Djokovic was left in tears after his taxing bid -- both physically and emotionally -- to become the first man since 1969 to win the year's four Grand Slam ended in a surprisingly one-sided loss to Russian second seed Daniil Medvedev.
Djokovic's bid for a record 21st major title that would have completed the calendar-year Grand Slam collapsed at the final hurdle.
He will face Russian Daniil Medvedev in Sunday's final.
The Serb is just two wins away from a 21st major that would complete a calendar-year Grand Slam.
The world No. 1 next faces Italian Matteo Berrettini in a rematch of the Wimbledon final.
Djokovic next faces Japan's Kei Nishikori, the 2014 US Open runner-up.
Djokovic took a first unsteady step toward completing the calendar-year Grand Slam as the Serb laboured into the second round.
The recurring issue flared up at the French Open, where he fell to eventual champion Novak Djokovic in the semi-finals.
The Serb pulled out of Saturday's mixed doubles bronze medal match with a shoulder injury.
Zverev became the first German man to make an Olympic tennis final since Tommy Haas won silver at the Sydney Games in 2000.