Down, but could injured Djokovic also be out in Paris?

Champion Novak Djokovic is fearful of whether he'll be fit to play in his quarter-final at the French Open after pulling off another great five-set escape despite a knee injury that required him to take anti-inflammatory pills just to finish the match.

On his return to action after a 3am finish in the previous round, Djokovic had to demonstrate all his astounding resilience once more to outlast Argentine Francisco Cerundolo 6-1 5-7 3-6 7-5 6-3 on Monday.

Hailed by Cerundolo as "really, the toughest guy ever to play tennis", Djokovic admitted he was "maybe three or four points away from losing".

He said he had considered quitting when he tweaked his right knee, which he revealed had already been a problem for him in the build-up to Roland Garros.

Novak Djokovic celebrates his hard-fought fourth-round win over Francisco Cerundolo. (AP PHOTO)

Yet he battled on for his 370th victory at a grand slam, taking sole ownership of the landmark for most major match wins he had shared with Roger Federer.

It kept him on course for a record-breaking 25th grand slam title, but the 37-year-old - who wondered if his four-and-a-half hour third-round win over Lorenzo Musetti in the early hours of Sunday was a contributory factor to his woes - admitted he was uncertain whether he'd be fit for his last-eight clash with Casper Ruud.

"(The) good thing about the slam is that you have a day between that will allow hopefully the healing process to happen more efficiently for me," Djokovic said.

"That's it. I don't know what will happen tomorrow - or after tomorrow, if I'll be able to step out on the court and play."

He got through the match against Cerundolo, he said, after the doctor gave him the maximum dose of anti-inflammatories.

It was just in time as the serial champion trailed by two sets to one, and was down a break at 4-2 in the fourth, defeat beckoning.

Djokovic on his knees
Djokovic slipped and fell, complaining about the court, during his victory. (AP PHOTO)

Djokovic also had a sharp exchange with the courts' supervisor about the playing conditions.

"I'm telling you as a player, it's not OK," he said, to which the official responded that the ground staff felt the surface was fine.

"They know better than me the court is good or not?" Djokovic scoffed.

He elaborated at his post-match press conference.

"I survived. I won the match. Great. But will I be able to play next one?" Djokovic said.

"I don't know. I don't know the severity of the injury. But could have this injury been prevented? Possibly, if there was just a little bit more of a frequent care of the court."

He had taken a nasty tumble when his footing slipped, yet picked himself out of the dirt to earn another improbable success and set up a quarter-final with two-time finalist Ruud, who beat Thanasi Kokkinakis's US conqueror Taylor Fritz 7-6 (8-6) 3-6 6-4 6-2.

This was Djokovic's 40th final-set win in 51 five-setters over his matchless career, as he made it into a record 59th grand slam men's quarter-final.

His 11th five-set win at Roland Garros put him level with Gael Monfils and Stan Wawrinka in the Open era.

But the rest of the final eight, including Australia's Alex de Minaur, will only be encouraged by the prospect of the draw opening up should Djokovic fail to make it to his quarter-final on Wednesday.

De Minaur, who beat fifth seed Daniil Medvedev 4-6 6-2 6-1 6-3, will face German Olympic champ Alexander Zverev, who didn't seal his four-hour 11-minute, 4-6 6-1 5-7 7-6 (7-2) 6-2 win over Holger Rune until 1.40am on Tuesday morning.