Even if Nadal withdrew or failed to win a group match, Djokovic would still need to reach the final to claim the year-end ranking for a record-equaling sixth time - a feat only American Pete Sampras has achieved.
His participation will be vital to determining who ends the year as world number one. "I need to see how things evolve every single day", he said. "I will be happy to keep competing with them for a while, and then I will be happy to watch it on TV".
"Everything Roger does, he does 100 per cent, so I think he's 100 per cent behind Sascha otherwise he wouldn't sign him".
"I started (serving) yesterday, very slow", Nadal, who won this year's French and U.S. Opens to move to 19 Grand Slam titles, one behind Roger Federer's record, said.
There has always been hypothesis over who would step up to self-discipline Federer, Nadal and Djokovic, who agree with dominated men's tennis for 15 years.
"I am confident that I can be very competitive, but it's a tournament that you will play the top guys since the beginning, so you need to be 100 percent ready", said the 19 Grand Slams victor.
"What I like about the young guys like Matteo here qualifying for the first time, it goes away from just saying like: 'Oh, we have some talented players on the tour, '" Federer told reporters.
As the eight men gathered for their pre-tournament commitments on Friday, taking a boat ride from the London Eye to the tournament venue at the O2 in south-east London, the youngsters contrasted neatly with the old guard of 38-year-old Federer, 33-year-old Nadal and 32-year-old Djokovic.
Indoor hard court has never been Nadal's favourite surface and injury has often got in the way of his participation at the ATP Finals.
"Obviously that serves as a great motivation", he said.
Medvedev, 23, has been the trailblazer for the subsequent era this three hundred and sixty five days, a hit two Masters 1000 titles, reaching the expedient of the US Initiating and hiking to quantity four within the area.
The clash between Djokovic and Federer is likely to be the highlight of the group stage, and it will be a first meeting since their dramatic Wimbledon final in July.
This year has been very important from every point of view: mentally I managed to go through very complicated situations, besides handling new ones.
Federer defended his decision to pull out of January's ATP Cup - where he had been due to face Andy Murray - for family reasons and revealed he will not play a tournament between here and the Australian Open, although he does have a number of exhibitions lined up.