There was much hope that yesterday's match between the two players would make for a rare competitive encounter at The O2 arena, but the world No 1 Djokovic took full advantage of a Zverev capitulation to triumph 6-4, 6-1 in 67 minutes.
The big-serving American will face Alexander Zverev on Friday afternoon in Group Guga Kuerten and, with the round-robin format, he still has a chance of reaching the semi-finals of the competition.
Zverev won only one more game.
"The first set was extremely tough and John played a great tie-break", Cilic said. His friend, Kyle Morgan, a former National Football League player who had worked the American player, died unexpectedly on Monday night, and Isner posted a moving tribute on Instagram that ended: "You were a friend, the best friend I could ever ask for".
Djokovic won the first set 6/4 on a single break of the Zverev serve, thanks to his returning success off the German's hard delivery in which he totaled nine aces and a whopping seven double faults. 'Aggressive, taking the ball early, making sure he takes it to Novak.
The 21-year-old Zverev, who has a Tour-leading 55 match wins this year, went toe-to-toe with Djokovic from the baseline and looked to have the firepower to worry his opponent.
The net game always looked unsafe for Zverev and so it proved as he double-faulted to concede the set.More news: Sony teases PlayStation Black Friday 2018 discounts
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"Maybe he was a little bit sick or something like that", Zverev said. "I think it's not sustainable". Zverev is obviously the leader. "I'll see them in a few weeks when I finish up my season".
Tennis's world governing body the ITF was given the green light in August to launch a new Davis Cup format next year, culminating in an 18-nation finals week to be held at the end of November in Madrid. "Got to go out there and play", he said.
"Yeah, it's very weird, to think I've lost twice but still have a shot".
"I was fortunate to be around people who emphasised the importance of stretching for the longevity of the career. How long that's going to go for, we don't know".
Novak Djokovic fears staging a revamped Davis Cup finals and a revived ATP World Team Cup weeks apart could hurt the sport and result in creating two "average" events. The tournament has only just started.' Rusedski also spoke about Kevin Anderson's stunning win against Kei Nishikori: 'I thought Anderson was absolutely brilliant from start to finish.
Cristiano Ronaldo was in attendance, with Djokovic perhaps spurred on even more by the Juventus forward's presence.