Novak Djokovic suddenly pulled up on his knee at the beginning of the second set of his match against Francisco Cerundolo at the French Open. The world No. 1 cruised through the opener 6-1.

But there were concerns when he started shaking his leg and called the physio during the changeover, receiving a medical time-out.

Djokovic finished his previous match at 3.06am – the latest finish in French Open history – and had significantly less time to recover for the round of 16.

If Djokovic was tired from his last match, there was no telling in the first set as he stormed through for the loss of just one game. But trouble struck in the second set when the 37-year-old hurt his knee in the third game of the second set as Cerudolo moved him side to side and drew him forward.

He wasn’t pushed into an obvious awkward angle. But it quickly became clear that something was wrong when he started flexing his leg in between points. He then called the trainer as they changed ends with the 23rd seed leading 2-1 and had a rigorous medical time out, starting on the bench before laying on a towel to receive treatment on the floor.

“Worst possible scenario for Djokovic,” Jason Goodall said on Tennis Channel. “It didn’t even look like he did anything drastic in the penultimate point of the last game but he did go a lot of chasing.”

Paul Annacone replied: “Not one significant movement that looks like it triggered anything but they’re doing some serious investigation and let’s see.” As the medical time out ended, Goodall added: “He didn’t have much time to recover after a 3am finish and this is the worst possible scenario.”

Djokovic continued to struggle in the next game and kept shaking his knee out. He saved two break points to keep himself ahead. And in the next changeover, the trainer was back out to give the top seed a brief massage in the allotted time.