World number one Novak Djokovic is on track to claim a record seventh Australian Open title after progressing through the quarter-final stage of the tournament on Wednesday when his opponent Kei Nishikori retired with a quadriceps injury.

Battling through three five-set marathons during the tournament and spending almost 14 hours on court before Wednesday's match, the battle-worn Japanese veteran pulled out of the contest in the second set with the score 6-1, 4-1.

"Before the match, I was okay," Nishikori said. "Of course, I wasn't fresh but I thought I was going to be okay. After third game or fourth game when I was serving, my right leg felt heavy. After that I couldn't really bend my knees and couldn't jump up, so I decided to stop."

"I'm sure it comes from my past matches, It's tough to lose like this, but I'll try to keep my head up," the world number nine added.

Feeling for his fellow competitor after struggling with his own injuries for much of last year, Djokovic said, "We've played so many matches throughout our careers, especially here a couple of times years ago. I hope he can recover, and it's not something very serious."

Now moving into the semifinals against world number 31 Lucas Pouille, the Serbian said he's not overlooking the inexperienced Frenchman. "I'm looking forward for a battle," Djokovic said.

"It's his first semifinal at a Slam, but he has won against (Rafael) Nadal in New York some years ago, and he has taken a couple of scalps at the big tournaments, so he's not afraid to play his best on the biggest stages in our sport."

"I expect him to come out and be very confident in himself as he always is, and I'm looking forward to it."