Zverev became the first German in the semi-finals of the event since Rainer Schuettler
Alexander Zverev became the first German to reach the last four at the ATP Finals for 15 years as he beat American John Isner 7-6(5), 6-3 at the O2 Arena on Friday.
The 21-year-old edged a tight opening set before breaking in the eighth game of the second to close in on his second victory in the Gustavo Kuerten group and move on to a semi-final clash against six-times champion Roger Federer.
His victory meant the day’s second singles between group winner Novak Djokovic and Marin Cilic is academic — apart from ranking points and the $200,000 (155,798 pounds) on offer for each round-robin win at the year-ending showpiece.
Zverev became the first German in the semi-finals of the event since Rainer Schuettler lost in the last four to Andre Agassi in 2003 in Houston.
World number 10 Isner still harboured hopes of reaching the semi-finals despite losing his first two matches.
A straight sets victory and a win for Djokovic later would have seen him scrape into the weekend.
When he levelled the tiebreak at 5-5 with a thunderous forehand return it still looked possible, but Zverev responded with a superbly improvised pass on the next point before Isner dumped a forehand into the net.
Despite his hopes having gone, Isner stuck to his task in the second set but Zverev, still needing the victory to make sure of his semi-final berth, powered to his 56th match win of the year.
Zverev, the leader of the next generation of players expected to dominate the game in years to come, is the youngest semi-finalist at the tournament since Juan Martin del Potro reached the final in London in 2009.
Regardless of what happens in the day’s second singles Djokovic will play South Africa’s Kevin Anderson in the other semi-final on Saturday.