Russia's Danil Medvedev cracked a broad, winning smile after edging Germany's Alexander Zverev 6-3, 6-7 (3-7) 7-6 (8-6) in a tense ATP finals encounter Tuesday.

The pale and serious looking Medvedev missed two match points after struggling from 4-2 down to 6-4 up in what was a see-saw final tie break that he described post-match as 'amazing'.

"I wanted to do something brave at 6-5," he said allowing himself to smile of his double slip.

"But that was a match to remember, just amazing," said Zverev, who beat Poland's Hubert Hurkacz in his openiong round robin encounter.

With a large crowd at the Pala Alpitour stadium the blue hardcourts, soft blue lighting and blue kitted players added to the moody feel. Zverev was certainly brooding as he went down 6-3 in a first set that was tighter than the score suggests.

The giant German appeared angered by a line call late in the set, before chanelling his energy to power ahead and draw level at 1-1.

The win puts Medvedev top of the Red Group ahead of Zverev who took a walkover win in his first encounter over the injured Matteo Berrettini.

In the Green Group Novak Djokovic began his attempt to equal Roger Federer's record number of ATP Finals triumphs with a straight sets win over Casper Ruud on Monday.

Russian Andrey Rublev also impressed with a 6-4, 6-4 victory over 2019 champion Stefanos Tsitsipas and sits behind Djokovic in the standings by virtue of winning one game fewer than the 20-time Grand Slam champion. (AFP)