Novak Djokovic into Australian Open semi-finals after Kei Nishikori retires

Nishikori has retired from 21 matches in his professional career, including from five matches at Grand Slams

Six-time champion Novak Djokovic reached the Australian Open semi-finals after Japan's eighth seed Kei Nishikori retired injured in their quarter-final.

Serbia's Djokovic, 31, was leading 6-1 4-1 when Nishikori quit with a thigh problem.

Every time Djokovic has reached the last four in Melbourne he has gone on to win the tournament.

The top seed will face Lucas Pouille in Friday's semi-final, after the Frenchman defeated Milos Raonic.

Spanish second seed Rafael Nadal and Greece's talented 20-year-old Stefanos Tsitsipas will contest the other semi-final on Thursday.

Djokovic has not lost to Nishikori in their past 15 meetings, stretching back to the Japanese's victory in their 2014 US Open semi-final.

"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."

• 'It's not about being a man or a woman, it's about knowing tennis' - Pouille praises coach Mauresmo

• Williams knocked out by Pliskova after holding four match points

• Osaka dominates Svitolina to reach semis

This is what the doctor ordered - Djokovic

Djokovic will play his 34th Grand Slam semi-final - only Roger Federer (43) has played more

After beating Russia's Daniil Medvedev in a gruelling last-16 match on Monday, Djokovic said he had a back injury which he "hoped" to manage before facing Nishikori.

Any fears the 14-time Grand Slam champion may be hampered were quickly allayed.

Once he broke in the second game for a 2-0 lead, plus again in a sixth game where Nishikori coughed up two double faults for 5-1, it never looked like he would lose on Rod Laver Arena.

Nishikori had spent almost 14 hours on court in his four matches up to the quarters, over four hours more than the Serb.

And it showed as the Japanese player's hopes were ruined by injury.

He called a medical time-out at the end of a first set which he lost in 31 minutes, requiring treatment from a physio on his right quad before returning to the court with heavy strapping.

The former world number four lasted another five games - losing serve in two - but was clearly struggling and decided he could not carry on with less than an hour on the clock.

"This is what the doctor ordered for after two nights ago, not to spend too much time on the court," Djokovic joked.'

'I couldn't beat anyone with one leg'

Kei Nishikori spoke to the press after his withdrawal

Nishikori, 29, had spent almost 14 hours on court in his four matches on route to the quarters - over four hours more than Djokovic.

The four-time Australian Open quarter-finalist needed more than five hours to beat Spain's Pablo Carreno Busta in their last-16 match on Monday.

"Before the match I was OK, I wasn't fresh but thought I was going to be OK," Nishikori said.

"After the third or fourth game I was serving, I felt heavy in my right leg and couldn't bend my knees or jump up.

"So I decided to stop. I couldn't beat anyone with just one leg.

"I'm sure it comes from my past matches, especially the last match. I was moving a lot and wasting energy."

• Feature: Why tennis players are turning to pickle juice

• Live scores, schedule and results

• Follow the Australian Open on BBC TV, radio and online