I was getting frustrated with myself about that.
Cilic kept his hopes of a semifinal berth alive as he came through this year's first three-setter - improving his record at the tournament to a still dismal 2-9.
Djokovic, who replaced the injured Rafael Nadal at the top of the rankings last week, has stormed up the charts this year following elbow surgery and a period of indifferent form, winning Wimbledon and the US Open along the way.
Djokovic, who beat Isner in his opening group-stage match, faces Cilic in his last group match on Friday, while Zverev, who defeated Cilic to kick off his Finals campaign, takes on Isner.
World number one Novak Djokovic produced a rock-solid performance to dismantle the challenge of German firebrand Alexander Zverev at the ATP Finals on Wednesday. In short, any one of the four-even Thiem with two straight-set losses, had a mathematical chance of making the semi-finals.
The American posted an emotional tribute to Kyle Morgan, who passed away aged 35 after being reported missing two days ago.
Day four's action at the ATP Tour Finals sees the second set of matches from the Kuerten Group.
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.
Debutant Isner double-faulted to hand Cilic a decisive break in the eighth game of the second set and then did the same to gift Cilic a break at the start of the third.
"Maybe he was a little bit sick or something like that", Zverev said. He has won many Masters events already, I think three in his career, which is quite impressive. I'm feeling good. I think I have played two quality matches (here at The O2 Arena). Zverev is obviously the leader of that next generation.
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. "Got to go out there and play", he said.
Zverev missed out on a semi-final berth on his debut at the season-ender previous year and will need to beat John Isner on Saturday to have a chance of sticking around for the weekend.
Djokovic said: 'The match was great. Following his resurgence over the last few months, which has seen him win 24 of 26 matches - while exhibiting a 118.7% combined hold/break percentage - he looks like he's getting towards his best levels from years gone by, which is extremely ominous for the rest of the tour.