LONDON—Alexander Zverev set up a semifinal with Roger Federer at the ATP Finals by defeating John Isner 7-6 (5), 6-3 on Nov. 16.
Zverev is the youngest player at 21 to reach the semifinals since 2009 and the first from Germany since 2003.
Isner and Marin Cilic, who plays group winner Novak Djokovic in the late match, were eliminated by the result. Djokovic will face Kevin Anderson in the other semifinal on Saturday.
Isner needed to win in straight sets to have any chance of advancing and almost took the lead when he held a set point in the 12th game of the opener. Zverev saved it with an ace and played a faultless tiebreaker to eliminate the American.
In the second set, Zverev converted his only break point of the match to lead 5-3 at the O2 Arena.
Reaching the last four in London is the latest achievement of a career-best season for Zverev, whose victory over Isner took him beyond the 55 wins he achieved last year.
Both players utilized powerful serves to hold comfortably but Isner came up with a crushing forehand return winner to bring up the first break point at 5-6.
Having forced a tiebreaker, Zverev produced perhaps the most important shot of the match at 5-5 to dig out a deep Isner return and force a volley error to ensure it was he and not the American who earned set point.
Isner netted a forehand and, with his chance of qualification gone, wilted in the second set as quality Zverev returns secured the only break of the match.
By Sam Johnston