Zverev missed out on a semi-final berth on his debut at the season-ender a year ago and will need to beat John Isner on Saturday to have a chance of sticking around for the weekend. "Especially in the second set he wasn't close to his best".
Novak Djokovic overcame an illness - and Alexander Zverev - to earn a second straight win at the ATP Finals.
"I don't think it was breathtaking tennis from both of us, but a win is a win", said Djokovic.
Nevertheless, following his 6-4 6-1 win, the seventh straight-sets singles result in seven matches so far at London's 02 Arena, Djokovic insisted: "I think there is a lot of quality in the new generation".
Adding to concerns about scheduling is the International Tennis Federation's revamped Davis Cup tournament - set to feature 18 national teams fighting for the title over one week of competition - being scheduled to take place at the end of the season, starting from November next year. I'll have to maximally prepare (for the match). That allowed me to step in. "I kind of enjoyed the time off but I missed this arena and I'm glad to be back".
"I thought I played well from midway through the second set, I started to swing through the ball but I haven't served that great". Djokovic saved it with a service victor but then double-faulted to give the 21-year-old another opportunity. With Cilic serving for the set at 5-3, Zverev broke back before holding serve with an ace to level at 5-5.
Mets' deGrom gets Cy with record-low wins; Snell takes AL
It had only been done four other times since 1969: by Greg Maddux twice (1994-95), Dwight Gooden (1985), and Nolan Ryan (1981). DeGrom started 32 games and finished his incredible campaign with a 1.70 ERA , 260 strikeouts and a 10-9 record.
Despite being widely-tipped as a future Grand Slam champion, Zverev has yet to go beyond the quarter-finals at any of the four majors. And a double-fault from Zverev in the next game gifted Djokovic the set after 48 minutes.
The high-quality encounter reached a crescendo when Djokovic pummelled Zverev into submission in the fourth game of the second set.
That was the signal for Djokovic to sprint to victory and he broke again in the sixth game before completing an impressive statement of intent.
The Croatian fifth seed had lost nine of his previous 10 matches at the Finals, and he looked in danger of another letdown when he dropped the first set against Isner.
His ability to play the percentages and churn out wins in third gear has been a key component in his rise to one of the all-time greats and that was all he needed on Wednesday.
Cilic kept his hopes of a semi-final berth alive as he came through this year's first three-setter - improving his record at the tournament to a still dismal 2-9.