Novak Djokovic has been given quick passage through to the semi-finals at the Australian Open after Kei Nishikori was forced to retire mid-match with a leg injury.
Clearly struggling, Nishikori was unable to keep up with the top seed, and he withdrew down 6-1, 4-1 in the second set.
Djokovic will next face France’s Lucas Pouille for a place in the final and a chance to earn a record-breaking seventh Australian Open title.
Nishikori had spent nearly 14 hours on court in the first four rounds of the tournament, including three five-set matches.
His last match prior to facing Djokovic had been a five-hour, five-set marathon effort against Spain’s Pablo Carreno Busta on Tuesday.
It was quickly apparent on Rod Laver Arena that the eighth seed was struggling physically against Djokovic.
His movement seemed sluggish, and he was being moved around by the number one seed.
Djokovic quickly broke for 2-0, then held serve, before Nishikori managed to hold for the first time in a seven-minute game to make it 3-1.
The 14-time major title winner was piling the pressure on his opponent, and Nishikori was unable to match Djokovic’s intensity.
He was broken again for 1-5, and Djokovic served out to love, ending the set in 31 minutes.
Nishikori had made 18 unforced errors and two double-faults in the opening set.
The Japanese star had an injury time-out after the first set, and was given assistance for his right quad and right hip.
He emerged again, taped up, for the second set, and held his serve for 1-0.
But Djokovic was not in the mood to take it easy on his opponent, and his relentless game was proving too much.
Nishikori was broken to love in the third game of the second set, and the signs were not good for the 29-year-old.
He gave repeated glances towards his support crew in the box as the set wore on, and when he lost the fifth game with another unforced error to go two breaks of serve down, Nishikori came over to shake hands with Djokovic.
Djokovic was concerned for his opponent, but admitted he was happy with the unexpected way his match turned out.
“I love to battle, especially against Kei. We’ve played so many matches throughout our careers,” he told Channel Nine’s Jim Courier.
“We’ve played here a couple of times. I hope he can recover, I hope it’s nothing very serious.
“As they say, this is exactly what the doctor ordered for me after a match two nights ago, not to spend too much time on the court.
“I’ve had plenty of matches so far this year. I’m in another semi-final and I’ll do everything to get ready for that one.”
Pouille stuns Raonic to reach first Open semi
Earlier, Frenchman Lucas Pouille continued his unlikely run in Melbourne by beating in-form Canadian Milos Raonic 7-6 (7-4), 6-3, 6-7 (2-7), 6-4 to advance to a maiden semi-final appearance.
The odds were stacked heavily against the 28th-seeded Pouille on his arrival in Melbourne after he lost all three of his singles matches for France in the Hopman Cup and made a first-round exit in Sydney.
Pouille exited in the first round in his five previous appearances at Melbourne Park and his quarter-final match-up was against an opponent who he had been unable to take a set off in three meetings.
But he simply dominated the 2016 Wimbledon finalist, facing a single break point over the entire match while earning 14 against Raonic, who is famous on the men’s tour for his booming serves.
“I didn’t have to face a break point for almost three hours,” Pouille said.
“Even if I lost the third set, in my mind it was clear I had to stay focused on my service game, taking care of that, then trying to put as many returns as I can.
“I’m still leading two sets to one, so I don’t have to panic. I really needed to stay positive, still doing what I did great for two hours 30 minutes.”
Raonic has now lost three quarter-finals at Melbourne Park after exiting at the same stage in 2015 and 2017.
He sent down 11 more aces than his opponent but his serve was unable to get him out of trouble as Pouille won the crucial points.