Federer beat Kevin Anderson 6-4 6-4 to ensure he would be one of the eight participants at the event in London.
It will be the Swiss master's 12 appearance at the season-ending Finals, although he does not enter as one of the favourites after a poor season saw him drop to seventh in the world rankings.
The 32-year-old, who was a beaten finalist in London last year, is the most successful ever player at the Finals, winning three titles and boasting a 42-9 record.
Federer barely broke sweat to knock out Anderson.
He broke his opponent's serve in the first game and never looked back. Anderson's series of unforced errors made sure there would be no contest at Bercy, even though the world number 21 rallied from 4-1 down in the second set.
Nadal was never really troubled by Granollers and broke decisively in the 11th game of each set to set up a meeting with last year's runner-up Jerzy Janowicz of Poland, the 14th seed.
Nadal won 80 per cent of his service points but only needed to break serve twice to triumph in one hour and 42 minutes.
"I am happy about the victory today," said Nadal. "First match indoors against not an easy opponent. So it is a good victory, a positive one for me.
"The victories on days when you are not playing well have much more value than the victories on days that you played well.
"He did a lot of things well. He played short points. He went to the net a lot of times. I have to improve a lot for tomorrow. I hope to be ready to do it.
"Tomorrow will be a tough match. I had a very tough one against him in Montreal this year. I'm going to try to play aggressively, play with more intensity than what I did today."
10-Milos Raonic (Canada) beat Robin Haase (Netherlands) 6-3 6-4
5-Roger Federer (Switzerland) beat Kevin Anderson (South Africa) 6-4 6-4
6-Tomas Berdych (Czech Republic) beat Pablo Andujar (Spain) 6-2 7-5
Philipp Kohlschreiber (Germany) beat 11-Tommy Haas (Germany) 6-2 6-2
1-Rafa Nadal (Spain) beat Marcel Granollers (Spain) 7-5 7-5
Grigor Dimitrov (Bulgaria) beat 16-Fabio Fognini (Italy) 6-3 5-7 6-2
4-Juan Martin Del Potro (Argentina) beat Marin Cilic (Croatia) 6-4 7-6(3)
7-Stanislas Wawrinka (Switzerland) beat Feliciano Lopez (Spain) 6-3 3-6 6-3
15-Gilles Simon (France) beat Nicolas Mahut (France) 6-4 6-7(5) 7-6(3)
12-Nicolas Almagro (Spain) beat Ivan Dodig (Croatia) 6-4 6-3
13-John Isner (U.S.) beat Michal Przysiezny (Poland) 7-6(3) 4-6 6-3
3-David Ferrer (Spain) beat Lukas Rosol (Czech Republic) 6-0 2-6 6-3
- Sports & Recreation
- Marcel Granollers
- Rafael Nadal