Novak Djokovic (37) sparked concerns about his injured knee for the rest of his French Open title defence, with the top seed saying he briefly considered pulling out midway through his five-sets win over Argentine Francisco Cerundolo (25) on Monday.

Djokovic had to dig deep for a second straight match after a third-round marathon against Lorenzo Musetti and needed medication during a 6-1, 5-7, 3-6, 7-5, 6-3 win over Cerundolo that sent him to the quarter-finals after four hours and 39 minutes.

Djokovic said he had been experiencing slight discomfort in his right knee for a couple of weeks and the issue flared up as soon as he slipped and tweaked it during the third game of the second set.

"At one point I didn't know whether I should continue or not with what's happening. I got the medications and then, after the third set was done, I asked for more medications and got them," said Djokovic, who also had his knee worked on by a trainer.

"That was the maximum dose that kicked in... after 30 to 45 minutes, which was about the time at the end of the fourth set when things started to really improve for me. I started to feel less limitations in my movement.

"The whole fifth set was almost without any pain, which is great. But the effect of the medications will not last for too long, so I'll see. We'll do some more screening and tests and checkups tomorrow, as well.

"We've done some with the doctor now. Some positive news but also some concerns, so let's see tomorrow.

"I can't tell you more about it at the moment. I'm glad I was able to play fifth set... without feeling pain I felt for two-and-a-half sets. But you never know what will happen tomorrow."

Djokovic, who also took a tumble late in the contest, said drier conditions after a week of rain had affected the clay.

"The injury that I had today with the knee happened exactly because of that, because I slipped and slid a lot," he added.

"Everyone slides on clay, but I slipped way too many times. That is quite unusual. I do, because I have an aggressive kind of movement, dynamic change of directions. It's normal for me.

"I've slipped and fallen on the clay many times in my life, and on grass as well, but this was just too many times today."

Djokovic said his team were having a conversation with tournament officials about why his requests to have the courts swept more frequently were being denied.

Up next for him is seventh seed Casper Ruud in a rematch of last year's final.

"Today I injured myself. I survived. I won the match. Great. But will I be able to play next one? I don't know," he added.

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