"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.
These numbers give decent insight into Cilic's struggles against Djokovic throughout his career, with the world number one having considerable joy on return against the Cilic serve, and it will certainly take a huge increase in level from Cilic for him to walk off court tonight with a victory.
Those group positions have the potential to be pretty significant, with Anderson now facing a more hard semi-final, in the shape of Novak Djokovic, while Federer now is up against the second place in the Kuerten Group.
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.
"The first set was extremely tough and John played a great tie-break", Cilic said.
Federer knows he has to lift his game and he will lift it.' Federer felt motivated by the whole scenario: 'Today, when highlights were showing from the last 10 years here at the O2, I really reminded myself again what a pleasure and what a privilege it is to play in this arena.
Djokovic netted a routine forehand when serving at 4-4 30-30, offering Zverev a break point that he did not convert. 'Aggressive, taking the ball early, making sure he takes it to Novak.
The 21-year-old German was playing his 74th Tour match of the season and after staying with the world No. 1 for nine games he collapsed to a 6-4 6-1 defeat. Here, he won just 11 points in a 28-minute set, leaving Djokovic to advance his cause at the O2, 6-1.
"Personally I'm still not thinking of the number 100". Zverev is obviously the leader. "I wish I could have given them a bit more to cheer for".
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.
"You have to say Roger is the favourite". I'll try on Friday.
"I mean, there's definitely some things I could have done better".
The evening session, of course, features two more over-30s in Cilic and Isner, but now the criteria had shifted a little with Djokovic's one-sided scoreline.
Cristiano Ronaldo was in attendance, with Djokovic perhaps spurred on even more by the Juventus forward's presence.