Defending champion Novak Djokovic says he does not know if he will be fit enough to play his French Open quarter-final after blaming the "slippery" Roland Garros clay for aggravating a knee injury.

Djokovic, 37, showed his supreme powers of recovery once again to come through a five-set marathon against Argentine 23rd seed Francisco Cerundolo in the fourth round on Monday.

The world number one was hampered by the injury before winning 6-1 5-7 3-6 7-5 6-3.

Djokovic will face Norwegian seventh seed Casper Ruud, who the Serb beat in last year’s final, in the last eight on Wednesday.

"At one point I didn’t know if I should continue," said Djokovic.

"I don't know what will happen tomorrow or if I'll be able to step out on the court and play. I hope so. Let's see what happens."

For the second time in three days, the 24-time major champion fought back from a two-sets-to-one deficit.

The fourth-round match with Cerundolo started at about 4pm local time on Monday, little over 36 hours after his previous contest against Lorenzo Musetti finished at 03:07 on Sunday morning.

But Djokovic still had the mental and physical resilience to win another gruelling encounter lasting four hours and 39 minutes.