No.3 seed Zverev started well but was quickly overwhelmed as errors started to dominate his game. "That's the issue. But I've said it before", Zverev, whose 55 match wins this year is more than any other player, told reporters.
Zverev nailed a trademark backhand victor down the line to earn five more match points and converted the first with a powerful first serve that Cilic could only fend into net. With the markets pretty adept at pricing up matches featuring two high profile players, we've been forced to pick our spots this week, but I'd much rather that approach when the situation dictates.
The merciless Djokovic held serve before breaking Isner once more with an inch-perfect backhand passing shot on match point.
In that match in Shanghai, Djokovic needed just one hour to break Zverev's serve four times without facing a break point himself, 6-2.
The manner of Zverev's collapse set the tone of what was to follow.
Zverev had clearly read up on the scenarios to reach the semi-finals and waved his arms aloft in the air, knowing that even if he went on to lose Marin Cilic would need to beat world No. 1 Djokovic for the German to be eliminated.
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Zverev knocked Isner out of the season-ending tournament earlier this afternoon with a 7-6 (5), 6-3 victory.
John Isner said it had been "very difficult" to focus on his second match at the ATP Finals against Marin Cilic after learning of the death of his close friend and former strength coach Kyle Morgan. "I honestly don't think it's good for the sport". "For that, I have to play good tennis". His sole win during the group stage action past year, however, came over Cilic.
It seemed to knock the stuffing out of Zverev, who was broken in the fourth game of the second set, and again to love in the sixth. If he were to win both of those matches at the end of a long, tough season, his coming of age would be near its apex.
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. To do that point in, point out it's very hard to do against the best.
In stark contrast to second seed Roger Federer, who produced an error-strewn performance in defeat to Kei Nishikori on Sunday, Djokovic hit just six unforced errors and won 86% of points on his serve, making a mockery of the apparent challenging nature of the playing surface.