Zverev books semifinal date with Federer with straight sets win at ATP Finals
The 21-year-old German is the youngest player to reach the semifinals at the season-ending tournament since 2009 and the first from his country since 2003.
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 twelvth game of the opener. Zverev saved it with an ace and then played a faultless tiebreaker to eliminate the American.
Zverev converted his only break point of the match to take a 5-3 lead in the second set at the O2 Arena.