World number one Novak Djokovic is through to the semi finals of the Australian Open after opponent Kei Nishikori was forced to retire.

The 31-year-old Serb was 6-1 4-1 up against his Japanese rival before Nishikori quit with a thigh injury.

Djokovic himself entered the match with a back problem stemming from his gruelling last 16 game against Daniil Medvedev on Monday, and was hoping to manage the condition throughout the match.

However, the troubles for Nishikori meant Djokovic never needed to get out of second gear to win the first set, after which Nishikori called a medical time out.

He returned to action for the second set with the right quad heavily strapped up but lost serve in two of the five games and retired just before the end of the second set.

And while Djokovic was pleased to get through to the semi finals he was disappointed he could not put on more of a show.

“I love to battle especially against Kei, we have played so many matches in our careers,” Djokovic said.

“I hope he can recover and it is not something very serious that will take too long.

“I am sorry for everyone not having a full match tonight but thank you for showing up.”

Djokovic is set to face Frenchman Lucas Pouille in the semi-finals on Friday after he triumphed in four sets against Milos Raonic.

Djokovic’s extra freshness might stand him in good stead for victory and he is 1.05 to come out, with Pouille 9.50 to cause an upset.