It was highly competitive with Zverev more than holding his own, and even threatening to break the World No. 1, but his failure to do so belied a worse fate, as he double-faulted away the set for Djokovic to take the lead.
Zverev, Cilic and Isner can all still join Djokovic in the semis, and calculators might be needed on Friday.
Isner wrote an emotional post on Instagram last night, saying: 'It's hard to explain how much I loved you and how instrumental you were in making me who I am today.
Zverev played a defensive forehand slice to set up a chance to finish the point with a backhand lob, but his attempt drifted just wide and Djokovic held on for 5-4.
Zverev has one of the most piercing serves in the circuit and has clocked speeds over 130 miles per hour even on second serves.
Novak Djokovic sent out a warning to his rivals at the ATP Finals on Monday, brushing aside big-serving John Isner 6-4, 6-3 to launch his bid for a record-equalling sixth ATP Finals title in style. "Then, I guess, you have mentally quite an edge", Djokovic, who has won 33 of his last 35 matches, said.
Number of missing in California fire doubles to more than 600
In a post on Twitter, she says her family has "found our answer" and asks for prayers as they get through this "very hard time". Scientists largely attribute the devastating blazes to climate change which has caused a prolonged drought in the state.
As in 2014, the first seven singles matches of the tournament have been completed in straight sets. I didn't serve that great and he obviously made a lot of unforced errors that allowed me to win probably easier than expected and especially in the second set, he wasn't close to his best. 6-1, and would go on to win his 32nd Masters title.
Zverev gave Djokovic a set point with a spate of errors in the next game, and conceded it with a double-fault.
Cilic's revival keeps him in with a chance of qualifying for the semi-finals, while American eighth seed Isner still has an outside chance of advancing despite losing his first two matches.
Rather like the Hewitt Group already discussed, the Kuerten Group also has a high likelihood of numerous last-round qualification permutations following the evening match, with Marin Cilica [1.57] favourite to get the better of John Isner, which again, I do make pretty accurate.
John Isner of the USA reacts after losing his group stage match against Croatia's Marin Cilic.
Djokovic said: 'The match was great.
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.
He wasn't close to done: "These are Angry People, including the highly conflicted Bob Mueller, who worked for Obama for 8 years". Whitaker is now overseeing the Mueller probe, which had previously been overseen by Deputy Attorney General Rod Rosenstein.
Speculation from news organizations like Reuters are reporting that the company to which Lumentum is referring to is Apple . Shares in Lumentum fell almost 30 percent, dragging down shares of other suppliers and chipmakers.
Six more people were confirmed killed in the Camp Fire in the north of the state, taking the toll there to 29. "You don't know if she's safe". "I didn't have time", he added . "She's a strong, smart woman".
Paradise, home to some 27,000 residents, has become the ground zero for the wildfire tearing through Northern California. Further south, fires have reached the beach resort of Malibu, with a "major disaster" now declared across the state.
By the way, maybe because of Robbo he is not 100 per cent offside, with his heel or whatever, and before that somebody holds him. It's more about the performances, particularly in the first half, but I don't think it was enough".
Veteran Shaun Livingston , meanwhile, dismissed the heated exchange as "just team spirit". "Obviously, Dray had the turnover". The altercation did not get physical, but Green's vigorous defense of himself helped increase the volume in the room.
Brooklyn Nets star guard Caris LeVert landed wrong on his right leg during a game against the Minnesota Timberwolves . The Nets ended up acquiring with the 20th overall pick in a trade with the Indiana Pacers for Thaddeus Young.
Weeks after the summit, North Korea returned the remains of dozens of presumed US soldiers killed during the 1950-53 Korean War. Global weapons experts said the officials around Kim included a leader of the artillery corps of the Korean People's Army.
Annette Villelas was one of those angry voters. "The way he talks just to the public, it's not right", Villelas said. The upheaval comes as Arizona, long a Republican bastion, emerges as a swing state in presidential elections.
Kevin McCarthy elected House minority leader
Steve Scalise , the current No. 3 House Republican, who successfully vied for the position of minority whip in the new Congress. Charles Grassley was nominated Wednesday to a high-ranking position that would put him third in line for the presidency.
Holly Willoughby shares I'm A Celebrity picture
Declan Donnelly has joked about co-star Ant McPartlin's absence, as his new 'I'm A Celebrity' press photo has been unveiled. Just stuff for you, 'coz I can kind of film things, which you can't get... just swiping out another fly.
Celebrities offer thanks, condolences in death of Stan Lee
In a statement The Walt Disney Company, which bought Marvel Entertainment in 2009, saluted Mr Lee's "unmatchable" life and career. The dapper, friendly comic book genius continued to work into his 90s on numerous projects, including comics, films and DVDs.
United Kingdom says it has reached draft Brexit deal with EU
Leadsom said on Sunday that MPs would not accept a backstop which the United Kingdom can not leave without the EU's permission. Pro-Brexit Conservative MP Iain Duncan Smith said May's days are numbered as prime minister if she goes ahead with it.
Deepika Padukone and Ranveer Singh Wedding
Ranveer's stylist, Nitasha, took to her Twitter account to share an update of a ceremony that was held on Monday evening. Speaking of Deepika's wedding attire, it's said that Sabyasachi Mukherjee will be creating the bride's handsome looks.
Police arrest woman in Australian needle-in-strawberry scare
The woman was arrested and charged with seven counts of contamination of goods under Section 238 Criminal Code on Sunday evening. Earlier, police spoke of the challenges investigators faced as they tried to figure out the source of the contamination.