World number one Novak Djokovic took to Instagram to congratulate Alexander Zverev for winning the 2021 ATP Finals after the latter defeated Daniil Medvedev 6-4,6-4. "Alexander Zverev had an incredibly tough year on and off the court...he is such a great guy," Djokovic captioned his Instagram post. Zverev defeated Djokovic in the semi-final of the tournament on Saturday.