Nishikori retirement sees Djokovic through to semi-finals
The world number one will face Lucas Pouille in the last four of the Australian Open after Kei Nishikori retired during the second set.
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.
Djokovic said a couple of fitness issues of his own had surfaced after a fourth-round contest with Daniil Medvedev which finished in the early hours of Tuesday morning, but it was his Japanese opponent who felt the strain of his exploits at Melbourne Park.
The top seed's bid to become the only man to win the first major of the year seven times was given a huge boost, as he spent just 52 minutes on court.
Djokovic had won all but two of his 17 encounters with the world number nine and there was yet more misery for Nishikori, who toiled from the start in the last quarter-final.
The 29-year-old, who twice came from two sets down to move into the last eight, netted a backhand to go 2-0 behind and it soon became apparent he was not moving freely.
Djokovic guided a simple backhand winner down the line for a double-break and a 5-1 lead, then served out the opening set in only 31 minutes.
A hampered Nishikori - who made 28 unforced errors in the shortened match - was broken to love to go 2-1 down in the second and ended the match prematurely after failing to hold again.