The in-form German player registered a straight-set victory over Daniil Medvedev to reign supreme in Turin.
Daniil Medvedev won the match against Casper Ruud in straight sets and recorded an impressive score of 6-4 6-2.
The Serbian World No.1 defeated Cameron Norrie to finish the round-robin stage with a perfect 3-0 record.
The German will face World No.1 Novak Djokovic in the semi-finals on Saturday after the Serbian secured top spot in Green Group.
Andrey Rublev shocked the Greek sensation Tsitsipas with a 6-4, 6-4 victory to start his Nitto ATP Finals campaign.
The World No.2 downed Hubert Hurkacz in his opening match and backed that up against the German.
Medvedev has won 23 of his past 26 matches, with 19 of his triumphs during that run has come in straight sets.
The Russian will play against Novak Djokovic, Stefanos Tsitsipas and Casper Ruud in the Green Group.
Zverev, who famously won the 2018 Nitto ATP Finals title, will compete at the season finale for the fifth straight year.
The Serbian went 27-1 in major championships this year, winning titles at the Australian Open, Roland Garros and Wimbledon.