Both players have beaten Diego Schwartzman but lost to Daniil Medvedev, meaning the winner here will progress to the semi-finals and the loser will be eliminated.
Whoever makes it through will meet Dominic Thiem in the last four.
When and where is the match?
The ATP Finals match between Djokovic and Zverev is on 20 November at The O2 Arena in London. The match gets underway at 2pm local time.
ATP Finals: Djokovic thrashes debutant Schwartzman
ATP Finals: Nadal off to flying start with win over Rublev
ATP Finals: Thiem wins three-set battle against Tsitsipa
How can I watch on TV and online?
The competition is available live on Amazon Prime, BBC Two, BBC iPlayer, BBC Sport website and mobile app.
When are the matches?
Sunday 15 November
Thiem def. Tsitsipas 7-6 4-6 6-3
Nadal def. Rublev 6-3 6-4
Monday 16 November
Djokovic def. Schwartzman 6-3 6-2
Medvedev def. Zverev 6-3 6-4
Tuesday 17 November
Thiem def. Nadal 7-6 7-6
Tsitsipas def. Rublev 6-1 4-6 7-6
Wednesday 18 November
2pm-4.30pm: Zverev def. Schwartzman 6-3 4-6 6-3
8pm-11pm: Djokovic vs Medvedev – Amazon Prime
Thursday 19 November
2pm-4.30pm: Rublev def. Thiem 6-2 7-5
8pm: Nadal def. Tsitsipas 6-4 4-6 6-2
Friday 20 November
2pm-4.30pm: Djokovic vs Zverev, Amazon Prime, BBC Two, BBC iPlayer, BBC Sport website and mobile app
8pm: Medvedev vs Schwartzman, Amazon Prime
Saturday 21 November – semi-finals
2pm-4.30pm: BBC Two, BBC iPlayer, BBC Sport website and mobile app
8pm-11pm: Amazon Prime
Sunday 22 November – finals
6pm-9pm: Amazon Prime, BBC Two, BBC iPlayer, BBC Sport website and mobile app