Schwartzman missed the chance to book his place when he was beaten 6-3 6-1 by third seed Daniil Medvedev in the French capital, giving Carreno Busta a glimmer of hope.
The Spaniard needed to win the title in Paris and then pick up more points at the Sofia Open next week to pip Schwartzman, and his hopes were still alive when he won just his second set in seven matches to lead against countryman Nadal.
But the world number two secured a crucial break of the Carreno Busta serve in the 12th game of the second set and ran away with the third to triumph 4-6 7-5 6-1.
Schwartzman is the eighth and final player to qualify for the season-ending tournament at London’s O2 Arena, joining Nadal, Novak Djokovic, Dominic Thiem, Medvedev, Stefanos Tsitsipas, Alexander Zverev and fellow debutant Andrey Rublev.
Nadal has never won the ATP Finals or the Paris Masters, and he feels he will have to return better if he is to continue his winning run against Alexander Zverev in the last four.
Speaking after his victory over Carreno Busta, Nadal said: “Of course I have to give him a lot of credit that he did a lot of things very well, played very aggressive.
“But, talking about my side, that’s the thing that really matters to me, I need to return better. My serve is working well, I think, so is the rest of the game more or less, I am playing better and better. I need to return better if I want to have chances to keep going.”
Fourth seed Zverev continued his strong form with a 6-3 7-6 (1) victory over Stan Wawrinka.
Meanwhile, it was officially confirmed that Djokovic will finish a year ranked world number one for the sixth time, equalling Pete Sampras’ record. At 33, Djokovic is the oldest man to achieve the feat.