World number one Novak Djokovic, bidding for a record-equalling sixth title at the event, is in the Tokyo group with Russian Daniil Medvedev, Germany's Alexander Zverev and Argentine first-time Diego Schwartzman.
World number two Nadal's fate will rest on his final group match against Tsitsipas.
The ATP Finals, in its 50th edition, is being staged in London's O2 Arena for the 12th and last time before it moves to Turin and sadly plans for a vibrant farewell party in front of the usual sell-out crowds has been scuppered by the COVID-19 pandemic.
Rafael Nadal's hopes of winning the year-ending ATP Finals for the first time live on after the Spaniard beat defending champion Stefanos Tsitsipas 6-4, 4-6, 6-2 on Thursday.
The tournament, staged in 11 cities across Europe, will feature 24 of the best teams from June 12.
Djokovic was soundly beaten by Zverev in the 2018 final, a match played out in an electrifying atmosphere in front of 18,000 fans. This time the arena was empty because of the COVID-19 lockdown, a surreal backdrop for a match with so much riding on it for both men.