While the O2 Arena has been a happy hunting ground for Djokovic with four of his five ATP Finals titles arriving there, the same can not be said of four-time qualifier Cilic.
Using a tissue to blow his nose between points, Djokovic was able to beat Zverev 6-4, 6-1 on Wednesday for his second straight victory of the tournament.
It was a close contest, with both players delivering a flawless performance, but Zverev edged the 1st set 7-6.
Djokovic also said fans should be patient with the likes of Zverev and fellow youngsters like Croatia's Borna Coric and Greece's Stefanos Tsitsipas as they seek to make their breakthroughs in the sport's biggest events.
The five-time champion saved two break points at 4-4 in the opening set and then broke Zverev in the following game to take the lead.
"My results here have been pretty good in the last 10 years, so I can't complain too much", said Djokovic, who has won four titles at the London venue.
Zverev played a defensive forehand slice to set up a chance to finish the point with a backhand lob, but his attempt drifted just wide and Djokovic held on for 5-4.More news: Mandy Moore Shares Emotional Message After Private Weekend Wedding
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Errors were starting to creep into Zverev's game and his first serve percentage began to fall as Djokovic increased the pressure at 5-4.
With Zverev having defeated Cilic in the opening match of the round-robin, all three players still have a chance of reaching the semifinals for the first time.
Cilic would have been kicking himself after allowing a first set that he initially dominated slip from his grasp.
"If I break him at 4-4, we'll see where the match goes".
The writing was on the wall for Isner when Djokovic broke serve at 3-3 in the second set - the Serb roaring his approval.
'I played well mid-way through the second set and started to swing through the ball.
With such a low figure count in full against the first racket of the world American could not, Djokovic made three breaks, and on his serve and lost only six points.
"It's hard, very difficult", Isner said. "Got to go out there and play", he said.