Spaniard Pablo Carreno Busta has questioned Novak Djokovic's physical struggles early on during their French Open quarter-final on Wednesday.

The Serb lost the first set, but recovered to secure his place in the last four at Roland Garros.

"Every time it gets complicated he asks for assistance - he's been doing this for a long time," said Carreno Busta.

"I don't know if it's something chronic in his shoulder or just mental, but he didn't put me off.

"I don't know, maybe it's the pressure or something that he needs to do it. But he continues playing normal, no? I don't know if he's in pain really or he has mental issues. Ask him."

After his victory, Djokovic, 33, said he had been suffering from neck and shoulder problems.

"A few things happened in the warm-up," said the top seed, who faces Greek fifth seed Stefanos Tsitsipas on Friday in their men's semi-final. "I had to deal with those physical issues coming on to the court. As the match went on, I felt better, didn't feel as much pain."

He added: "I had some neck issues and some shoulder issues. I'll just say that. I mean, I don't want to get really too much into it. Obviously I'm still in the tournament, so I don't want to reveal too much."