Novak Djokovic reached the semi-finals of the Australian Open when Kei Nishikori retired due to a quad injury in the second set. 

Nishikori had been taken to a fifth set three times on his way to the last eight and that appeared to catch up with him on Rod Laver Arena on Wednesday. 

The struggling eighth seed called for treatment on his right leg at the end of a first set which Djokovic won 6-1 and was trailing 4-1 in the second when he shook hands with the world number one.