Tennis news - Novak Djokovic into Australian Open semis after Kei Nishikori retires with injury

Nishikori, seeded eighth in Melbourne, was evidently struggling from the start of the match and quickly slipped to 6-1 in the first set.

At 4-1 down in the second the Japanese star offered a handshake to Djokovic, who enjoyed a comfortable passage to the last four where he will face France's Lucas Pouille.

"I wish Kei a speedy recovery and I’m sorry for everyone for not having a full match tonight," Djokovic said on Eurosport.

"This was my first Grand Slam trophy back when I was 20 years old. That opened a lot of doors for me. I have great memories from this arena and the fans that have supported me over the years."

Nishikori had played over 14 hours at the Australian Open - coming through an epic five-setter against Pablo Carreno Busta in the quarter-finals, and in his first-round and second-round matches too.

He was clearly struggling after a handful of games in the evening match at Rod Laver Arena and had a medical time-out after losing the first set. He grimaced as a trainer worked on his right thigh and pulled out of the match after Djokovic won the fifth game. He hobbled off the court and up the stairs as he returned to the dressing room for further treatment.

Although appearing untroubled by any injury niggles, Djokovic was grateful for the short match, having complained of a sore back and a "couple of issues" after battling through a taxing four-set clash against Russian Daniil Medvedev in the previous round.

"As they say this is exactly what the doctor ordered," he told Jim Courier in their on-court interview. "I've had plenty of matches so far this year .. And I'll do everything to get ready for that (next) one."

Djokovic is now eagerly awaiting his 34th Grand Slam semi-final.

"I’m looking forward to a battle," he said of Pouille. "He has made a couple of scalps at the big tournaments and he’s not afraid to play his best at the biggest stages in the sport.

"I’m expecting him to come out and be confident and I’m looking forward to it."