Djokovic's outstanding serving, mighty forehand and all-court defence delivered him a ninth straight win over Tsitsipas in their Red Group match.
Djokovic broke the Greek in the opening game and rode that advantage to capture the first set in a match that came just two weeks after the Serbian beat Tsitsipas at the Paris Masters, a tournament that is also played on indoor hard courts.
Tsitsipas finally got his lone look at a breakpoint opportunity while leading 2-1 in the second set but Djokovic crushed an overhead to erase the danger.
Tsitsipas showed signs of life in the tiebreak when he battle back from 5-1 down to get to 5-4 but Djokovic pounded two precise serves to reach the finish line.
Djokovic will next play Andrey Rublev while Tsitsipas will face Daniil Medvedev in the second round of the round-robin stage of the season-ending tournament for the world's top eight players.