With Carlos Alcaraz out of the ATP Finals, Nadal and Tsitsipas have a very decent chance of finishing the year as number one.
Alcaraz was looking forward to playing at the ATP Finals but a muscle tear in his abdomen will keep him out for at least a month making it impossible for him to play at the ATP Finals. He sustained the injury in his match against Rune in Paris and that gives Nadal and Tsitsipas an even better chance of finishing ahead of him in the rankings.
There are many different scenarios where Nadal and Tsitsipas will finish ahead of Alcaraz and we'll lay them out below. Nadal will finish as number one if he wins the ATP Finals or if he reaches the final with a 3-0 round-robin record.
Tsitsipas on the other hand can finish number one if he wins the finals with a 3-0 record in round robin. Every other scenario including Tsitsipas and Nadal not making it out of the RR stage will see Alcaraz finish as number one.