There isn't much information about the two but it is known that they met each other in 2016 while they were in their teens.
In a free-hitting show on Margaret Court Arena, the Croatian blasted forehands and hammered backhands to end Rublev's time in Australia
Russia defeated Croatia in the final at Madrid to win the Davis Cup title after 15 years.
The 22-year-old bounced back from losing his opening round-robin match against Novak Djokovic by defeating Cameron Norrie and Andrey Rublev.
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.
They defeated Team World’s Reilly Opelka and Denis Shapovalov 6-2, 6-7, 10-3 to give their team an incredible 14-1 victory.
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.