The game ended with a score of 6:2, 6:7, 6:0 in favor of the 24-year-old Russian tennis player.
The match lasted 2 hours and 29 minutes.
Djokovic, who turns 35 in May, is the No. 1 in the world.
Rublev was seeded under the second number at a tournament in the capital of Serbia.
The prize fund of the tournament is more than €500,000.
Earlier, Daria Kasatkina spoke about the removal of domestic athletes from participation in Wimbledon.