The opening matches at the ATP Finals led to a debate over whether conditions at the O2 Arena were trickier than in past years.
Further errors from Cilic saw Zverev race to a 5-1 lead before a classy backhand victor and a unreturnable serve sealed the second-set tiebreak, as well as the opening win.
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.
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A forehand error from Isner confirmed the break, and the end of the contest.
A tired looking Cilic saved a match point to take it to another breaker, but his forehand continued to misfire and he again opted against challenging when his second serve was wrongly called out as Zverev wrapped up his 55th win of the season. "But the tournament isn't over", Zverev said. And it's very high-bouncing as well.
Zverev, the leader of the next generation of players expected to dominate the game in years to come, is the youngest semifinalist at the tournament since Juan Martin del Potro reached the final in London in 2009. I think [we've] played against each other only twice. That's the most important thing.
Zverev has always been talked about as one of the brightest talents of his generation and will be keen to make a statement in London after again flattering to deceive at Grand Slam level this season.
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Djokovic has enjoyed a fine renaissance in the last six months taking both the Wimbledon and US Open titles.
In the second set tie-break Cilic was left to rue a wasted challenge earlier in the set which meant he did not have one left to overturn another bad call. "I played very solidly".
Third seed Zverev has won three titles this year, including the Madrid Masters, but although he is often talked about as the leader of the next generation, his record in Grand Slams is poor and he will be desperate to make a statement in London.
An early exit for the Swiss veteran would deprive the tournament of star power, with world number two Rafael Nadal already missing through injury.
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