After the world No 5 won a crucial point in his match with Nicoloz Basilashvili at the Shanghai Open, he showed his delight with a fist pump.
"I just feel like the date of the Davis Cup is really bad, especially for the top players", said Djokovic, echoing the strong sentiments of Zverev a day earlier.
'He knows how to play, definitely.
"Probably the hardest-hitting player I have ever played against", said Zverev.
The Australian eased to a 6-4 6-3 win over Frenchman Benoit Paire as his late season run gains momentum.
It is his first third round appearance at a Masters series event, the highest rated tournaments outside the grand slams.
The world number five faced tough resistance from Georgia's Nikoloz Basilashvili in the match opener, but managed to seize the initiative to claim a two-set victory 7-5 6-4.
The 21-year-old will face an opponent younger than him in his next round when he takes on teenager Alex De Minaur.
The inaugural Davis Cup finals are slated for a week in Madrid in November 2019 as part of a revamp to the competition, but it has been contentious with players.
"I think all the top guys will say the same thing".
"Making a tournament end of November which is 10 days playing and competing, it's insane".
The global competition was previously played at various stages throughout the regular season but it will take place at the end of the campaign from 2019 - which in turn prolongs the season. "We'll see what happens", said the 37-year-old, who has been carefully managing his schedule since 2017 to wring the most out of his ageing body.