Novak Djokovic will be ranked world No.1 next week for the first time in two years after Rafael Nadal's withdrawal from the Paris Masters.

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.

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 turned his form around, winning four of the past five tournaments he has entered, including Wimbledon, the US Open and most recently the Shanghai Masters.

He will play Damir Dzumhur in the third round in Paris, an event he has won a record four times.

Nadal, who has not played since a knee injury curtailed his US Open campaign in the semi-finals, pulled out of the final tournament of the regular season due to an abdominal injury.

"The last few days I start to feel a little bit the abdominal, especially when I was serving," the Spaniard said.

"The doctor says that is recommended to not play, because if I continue the abdominal maybe can break and can be a major thing."

Nadal did not reveal if he will play in next week's ATP Tour Finals in London.

Roger Federer, who won his 99th career title at the Swiss Indoors last Sunday, advanced to a third-round meeting with Fabio Fognini after big-serving Milos Raonic retired before their match with a right elbow injury.

On the court in Paris, Kei Nishikori and John Isner kept their hopes of gatecrashing the Tour Finals alive with victories.

Japanese Nishikori beat Frenchman Adrian Mannarino 7-5 6-4 while American Isner battled past Mikhail Kukushkin 6-3 6-7 (2-7) 7-6 (7-1).

Fourth-seeded German Alexander Zverev booked his place in the next round with a 6-4 6-4 win over Frances Tiafoe.

There were also wins for defending champion Jack Sock Kevin Anderson, Dominic Thiem, Grigor Dimitrov, Borna Coric, Diego Schwartzman and Malek Jaziri.

Roger Federer, fresh from his recent title in Basel, made it through to the third round without playing after second-round opponent Milos Raonic pulled out.