Federer dropped outside the top four for the first time in a decade this season, and also failed to reach a Grand Slam final this year for the first time since 2002.
However, he said that qualifying for the Tour Finals is the accomplishment of one of his goals.
"It feels great," Federer, now ranked sixth, told Sky Sports News.
"I'm so happy, it's been a tough year so this is a highlight because this was one of the big goals I set for myself this year so it's great to have made it."
The Swiss player will play Philipp Kohlschreiber in the next round in Paris.