The Serbian World No.1 registered a comfortable straight-set victory over the fifth-seeded Russian at the Nitto ATP Finals.

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.

The Russian will play against Novak Djokovic, Stefanos Tsitsipas and Casper Ruud in the Green Group.

Alexander Zverev led 5-2 in the deciding set but failed to convert his two match points, allowing Taylor Fritz to make a comeback.

Elsewhere, Alexander Zverev sent a warning to the rest of the field at the BNP Paribas Open by easily walking past Frenchman Gael Monfils in their fourth-round clash.

The 30th seed rattled the Greek player early on, but Tsitsipas worked his way back into the match to register a 2-6, 6-3, 6-4 victory in two hours and four minutes.

The 34-year-old, who has undergone two right hip surgeries in the past three years, also spoke about the adjustments made to his service technique after Wimbledon.

The ATP Finals 2021 will close out the men's tennis season at the end of the year and so far, only three of the eight spots in the tournament have been filled.

Inordinately long toilets breaks and medical timeouts at critical moments could soon be a thing of the past.