Given his history of injuries, Nadal listened to advice and did not play his second-round match against Fernando Verdasco on Wednesday.
"But in the last few days, I've started to feel the abdominal a little bit - especially when I was serving". I was checking with the doctor, and the doctor says it's recommended to not play.
The Serb will lead the ATP Rankings in men singles for the first time in two years, with Briton Andy Murray having taken the top honour from him in Paris in November 2016.
Should Nadal win, he would stay at the top, but also celebrate his very first Paris Masters title.
The 31-year-old struggled to come back from a lingering elbow injury with his ranking plummeting to No.22 in May this year. Djokovic was in total control and did not face a break point, breaking his opponent's serve three times.
"I have to figure out how to play Roger again, but I think I've been playing well the last couple of weeks so hopefully I can bring that again tomorrow", Nishikori said.
Over the course of the past decade, the ATP's "Big 4"-Rafael Nadal, Novak Djokovic, Roger Federer and Andy Murray- has collectively dominated at the Grand Slams and the majority of the ATP Masters events". But his record in Paris-Bercy is not great-the third round last year-and he has a tough opener in the shape of Lucas Pouille or Gilles Simon. He then skipped the Asia swing to recover, missing tournaments in Beijing and Shanghai. I really don't want that. So as I said, I can't be too ecstatic about it.
Nadal is optimistic his latest injury will pass, providing he does not rush back. It was me today.
Cilic, the fifth seed, secured his quarter-final ticket with a 7-6 (7/5), 6-4 defeat of Grigor Dimitrov. The Canadian had survived a three-tiebreak victory over Jo-Wilfried Tsonga in the first ound Tuesday night. I wanted to thank them, to salute them. He easily beat Fabio Fognini in straight sets.
Fourth-seeded Alexander Zverev saved all four break points he faced in beating Diego Schwartzman 6-4, 6-2.
Khachanov slid on his back and stretched out his arms in celebration after prevailing against John Isner 6-4, 6-7 (9), 7-6 (8) in a match in which the eighth-seeded Isner had 19 aces but missed two match points.
For Djokovic to surpass top-ranked Rafa Nadal when the next edition of the ATP rankings are released on Monday, he must outlast the Spaniard in the French capital this week. Djokovic next faces Damir Dzumhur.