Yet despite him not feeling 100 percent, it illustrated just how the Serb has become one of the all-time greats, mastering the ability to churn out victories in third gear.
Last year, the football star was in attendance to watch Spaniard Rafael Nadal beat Djokovic in the semi-finals of the Mutua Madrid Open.
Already guaranteed to end the year as world number one after winning Wimbledon and the U.S. Open, Djokovic is a massive favourite to complete an incredible resurgence from world number 22 in June with another title in London's docklands.
Zverev provided some fierce resistance in the opening set of their round-robin match, but as he so often does Djokovic played the percentages.
Day four's action at the ATP Tour Finals sees the second set of matches from the Kuerten Group. "I had three breaks of serve of John which is sometimes mission impossible, but I managed to be at the right place at the right time". I think also it seemed like Sascha was not feeling really great, definitely not playing well, especially from the back of the court.
The German eventually forced the first break points of the match with the score level at 4-4 but Djokovic held after a backhand lob from Zverev drifted inches wide.
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Police said they hope to find the man, but clarified that he is not in trouble, according to KTLA-TV . The suspect was described as a Hispanic woman with brown hair, approx.
"I don't think it was breathtaking tennis from both of us, but a win is a win", Djokovic, who hit only three winners in the first set compared to Zverev's 15, said on court.
A second tiebreak arrived and just as in the day's first shootout, it was Zverev who jumped into a lead against an edgy Cilic who double-faulted at 1-4.
The net game always looked unsafe for Zverev and so it proved as he double-faulted to concede the set.
"The match was great, obviously", said the 31-year-old.
Zverev looked deflated and despite holding serve for 1-1 he offered no more resistance as Djokovic won five games in a row.
'I had not served that well but he [Zverev] made a lot of unforced errors which helped me to win.