Zverev then played a nerveless tiebreaker to set up a title decider with either Novak Djokovic or Kevin Anderson, who will play in the other semifinal later Saturday.
Notwithstanding the oddness of the World Tour Finals - ostensibly featuring the top eight players in the world, but in effect featuring the top eight players in the world by ranking who are fit to play, and coming at the end of a gruellingly long season - lifting this particular trophy is a real milestone for Zverev, for two big reasons. It was a very happy moment for me that I won the match. "I think this is a pretty good stat, especially as he's the best returner we have in the game".
"It makes it so much easier for us to play so thank you for that".
Zverev booked a showdown against Federer with a straight-sets win over American John Isner.
Losing World Cup semifinalist England took some measure of revenge against Croatia with a 2-1 win against the team that dumped it out of this year's tournament in Russian Federation, thanks to a 85th-minute Harry Kane goal.
Anderson nicked one game back but it was a hopeless task and Djokovic put him out of his misery with a netcord on matchpoint to claim a 35th win in his last 37 matches - a run including the Wimbledon and U.S. Open titles to swell his collection of Grand Slam titles to 14.
Alexander Zverev showed his class on and off the court after defeating Novak Djokovic by lauding the world number one's remarkable year. "I will hope. I mean, I will do everything I can to get better, to compete with them always".
"He apologized to me at the net". The German man converted both of his break point chances, with Federer going one from three, and ultimately the match revolved around these five points, as well as the second-set tiebreak.
He then made no mistakes when serving out for the match, clinching the semi-final spot to set up a meeting with Roger Federer for a spot in Sunday's final. "The ballboy dropped the ball and it's in the rules that we have to replay the point".
Zverev, billed as the player most likely to lead the sport into a new golden era once the holy trinity of Roger Federer, Djokovic and Rafael Nadal have hung up their rackets, took full advantage of an off-key Serb to win 6-4, 6-3.
"I was making way too many unforced errors", Djokovic said. Definitely not mad at him. The Serb had already dealt with some illness at the Paris Masters where he lost to Karen Khachanov in the final. 'Health-wise I haven't been really ideal in the last three, four weeks', Djokovic admitted.
The young German has often been talked about as the leader of the next generation but despite having three Masters titles under his belt in his short career, he has a poor record at the Grand Slams. I've told him it is one thing winning a match or two and reaching a quarter-final, and another being successful at a major event like the ATP finals and Grand Slam.