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.

The top seed broke out into a beaming smile when he sealed victory, pointing to his chest and then to the court in celebration.

It was Djokovic's 370th victory at a Grand Slam tournament, putting him clear of his great rival Roger Federer.

"I don't know how I won," the three-time Roland Garros champion said.

Roland Garros clay 'screwed up' Djokovic's knee

While Djokovic had moved in a "positive direction" after straight-set wins in his opening two matches, the exertions of beating Musetti led to questions about his shape against Cerundolo.

Finishing in the early hours also reopened a long-running discussion about why tennis finishes so late and the impact on the welfare of the players.

Djokovic looked tetchy and uncomfortable from the start.

Before the match he was wearing tape behind his right knee and needed treatment on the same leg early in the second set, laying out on the court as the physio assessed him.

Djokovic was heard telling the physio that he had "screwed up" his knee, adding: "I'm slipping and sliding all the time."

He was given painkillers and called the physio to his chair several more times over the rest of the match.

"I had slight discomfort in the right knee for a couple of weeks but it was not an injury which was concerning - until today," Djokovic said.

"The late finish did not help the sleeping, the biorhythm and recovery, but I actually felt good in the circumstances.

"Then I slipped, one of the many times I slipped, and that affected the knee. I started feeling the pain.

"The reason why I continued was because I wanted to see if an extra anti-inflammatory was going to kick in and reduce the limitations - which is what happened."

How Djokovic avoided earliest exit since 2009

Write off Djokovic at your peril. So many times he has looked on the brink, so many times he has produced a logic-defying victory.

But even after seeing countless comebacks over the years, the manner in which he beat Cerundolo - at this stage of his career - was still remarkable.

Djokovic arrived at Roland Garros on the back of a turbulent season, having played below his lofty expectations and not reached a final, as well as parting ways with long-time coach Goran Ivanisevic.

Djokovic was breathing heavily and limping around the court as he looked set to suffer his earliest Roland Garros exit since a third-round defeat in 2009.

It would also have been his earliest exit at a Grand Slam tournament since the 2020 US Open – when he was defaulted after accidentally hitting a ball at a line judge.

But, after going a break down in the fourth and trailing 4-2, he fought back to level at 4-4 and struck again when Cerundolo served to stay in the set.

Now moving better and acting less demonstratively, Djokovic broke for 2-0 in the decider and it looked like he could race away to victory.

Cerundolo was not finished as he aimed to reach his first major quarter-final, though.

The 25-year-old Argentine instantly put the set back on serve, but Djokovic exerted more pressure to break again for 5-3 and serve out another memorable win.

Later, Ruud teed up a repeat of last year's final with a 7-6 (8-6) 3-6 6-4 6-2 win over American 12th seed Taylor Fritz.