Novak Djokovic was given an easy passage through to the semi-finals of the Australian Open when opponent Kei Nishikori retired.
The eighth seed had come through three five-set matches, including a five-hour epic against Pablo Carreno Busta in the previous round, and was struggling from the start.
He received treatment to his right thigh at the end of the first set but, after going 6-1 4-1 down, Nishikori shook his head and then the hand of Djokovic.
The world number one said: "I love to battle, especially against Kei, we've played so many matches.
"I hope he can recover, I hope it's not something that is very serious. I'm sorry for everybody for not having a full match tonight."
Djokovic had endured his own physical battle against Daniil Medvedev on Monday and admitted afterwards that he was feeling the effects.
"This is exactly what the doctor ordered," he said, "Not to spend too much time on the court. I'm in another semi-final and I'll do everything to get ready for that one."
