"I'm happy to play tomorrow". "I enjoyed it [and] I felt in terms of tennis better than one week ago".
Serbian Djokovic lost his top ranking to Andy Murray after the 2016 ATP Masters 1000 event in the French capital but has fought his way back to the summit this year.
"It's not the end of the season. Then I'll be able to speak more about it next time we see each other", Djokovic pointed out.
Federer's last appearance at the Paris indoor event was a third-round loss to big-serving John Isner in 2015. Rafa is in the race.
Asked on Tuesday if he was also previously invited, Federer said "they contacted me too, yeah".
The fifth-seeded Cilic beat ninth-seeded Grigor Dimitrov 7-6 (5), 6-4, needing six set points to win the first set.
"It was great to be here in Paris for a couple of days and practice with the guys", said Nadal who hasn't played since the US Open.
Cilic, who will guarantee making the cut for London if he downs Djokovic, was ending reigning ATP Finals champion Dimitrov's season.
The 16th-seeded Sock saved all four break points he faced in a 6-3, 6-3 win against Frenchman Richard Gasquet, while Zverev advanced 6-4, 6-4 over American Francis Tiafoe.
Later Thursday, Federer faces volatile Italian Fabio Fognini for a place in the last eight. He was fresh after Milos Raonic pulled out injured before their second-round match on Wednesday.
So far this week, there have been five retirements and two walkovers as the toil of the long season begins to bite. Nothing is said at the moment but we're just still trying to figure how we're going to make the schedule. Djokovic has occupied top spot for 223 weeks during his career and another extended stay beckons.
Isner's U.S. compatriot Jack Sock, who's the defending Paris Masters champion, also advanced with a 6-0 6-4 triumph over Malek Jaziri. His personal experience with the game made him an even more profound voice for the sport.
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