Zverev saved two break points before a double fault - his 18th unforced error of the opener - handed Djokovic the set.
Ronaldo got involved in ball kid duties while watching world number one Novak Djokovic's opening match against John Isner of America as the tennis ball bounced into the stands near the first row where he was sitting.
Novak Djokovic sent out a warning to his rivals at the ATP Finals on Monday, brushing aside big-serving John Isner 6-4, 6-3 to launch his bid for a record-equalling sixth ATP Finals title in style.
"I started to play better in the second set and he was making a lot of unforced errors, which helped me win".More news: George RR Martin's Wild Cards TV Series Now Headed To Hulu
From there Djokovic won the next three games, dropping only one point, as he wrapped up his second win of the week.
Yesterday's action in London saw two comprehensive straight-set victories in the Hewitt Group, with Kei Nishikori destroyed by Kevin Anderson, and Roger Federer easing past Dominic Thiem, to get his first victory of the tournament.
"The match was great, obviously", said the 31-year-old. Zverev then conjured up a match point with a sublime backhand victor at 30-30 at 5-4 but Cilic resisted and held serve with a strong serve down the middle. Sometimes you got to hope he's a bit off.
Djokovic broke twice more in the second set as he started to take more control although he was still far from his best.More news: Victims Sue PG&E Over Northern California Wildfire
The Serb is a red-hot favourite to draw level on six titles at the season finale with Federer, who faces an uphill task to reach the semifinals after his loss in his first round-robin match.
"I don't think it was breathtaking tennis from both of us, but a win is a win", said Djokovic.
The German No. 3 seed, who also defeated Cilic in their opening match of last year's season-ending tournament, was down a break of serve in both sets but came through 7-6 (5), 7-6 (1) at the O2 Arena.
Last year, the football star was in attendance to watch Spaniard Rafael Nadal beat Djokovic in the semi-finals of the Mutua Madrid Open.More news: Kyrie Irving Seems to Have Forgotten Al Horford is on the Celtics
Having failed to convert break points in Isner's opening two service games, Djokovic made no mistake when another chance came along, belting a forehand return for a victor and a 4-2 lead. "It worked out", Zverev, who vented his anger in the direction of coach Ivan Lendl in the early stages, said.