The world number nine Schwartzman earned his place in London's O2 Arena as world number five Roger Federer has chose to skip the rest of the season due to a knee injury. Schwartzman made the quarters of the Australian Open, the semis of the French Open, and the final of the Rome Masters.
Who will win as Nadal, Djokovic compete?
The 50th anniversary of the first staging of the event has been marked with the naming of two round-robin groups, Tokyo 1970 and London 2020.
Murray and Skupski needed to win the title and for Melzer and Vasselin to miss the final to snatch the last ticket to London.
Djokovic begins his campaign on Monday afternoon against the last to qualify this year, No8 Diego Schwartzman, and he had warm words about the Argentine he has beaten in all five previous meetings.
But make no mistake: At the age of 33-younger than Federer by six years-Djokovic has plenty more time and energy in the tank, and his ambitions are, if anything, only sharpened by the milestones of his great rival.
Nadal said there are a couple of matches at the venue which bring back good memories.
On the other side, Nadal still aims to win his first title at the ATP World Tour Finals, a trophy missing from his exquisite cabinet.
What will also give him confidence in his quest for a maiden Tour Finals title is that he twice finished runners up at the indoor venue. Though he did lose in the semifinals last week to Alexander Zverev in Paris.
The fifth game ultimately proved to be decisive.
Nadal was the top seed at the event a year ago, failing to make it out of the round robin despite a 2-1 record.
A close 7-6 2-6 6-7 loss in the final to Tsitsipas will leave Thiem hungry to go one better this time around.
Qualification for this year's event was based on ATP rankings instead of the Race to London standings due to the coronavirus pandemic.
The 23 year old reigning champion is making just his second appearance at the O2, after winning on his debut last year. After next week, the ATP Finals will no longer be played in London.
The man from Greece has however suffered from a leg injury of late, and most recently he exited the Paris Masters in his first match against world number 30 Ugo Humbert.
World number eight Rublev leads the ATP Tour this year in titles won with five and tour-level victories with 40.
Joining Nadal and Tsitsipas will be US Open champion Dominic Thiem, who has recovered from a foot problem, and Russian debutant Andrey Rublev.
Following this Nadal will face off against Rublev in the evening match.
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