Marathon man Nishikori has already spent 13hr 47min on court in Melbourne this year, more time than any other player, coming through three five-set epics in his four matches so far.
He has twice clawed back from the precipice of being two sets down, the latest against Pablo Carreno Busta on Monday that went to a deciding super tiebreak.
Djokovic, after coming through his own draining late-night slugfest against Daniil Medvedev, said he knows what to expect from the eighth seed.
“Well, Kei won another marathon match,” Djokovic said of the Japanese number one, who will be in his fourth quarter-final in Melbourne but is yet to reach a semi.
“Congratulations to him for fighting back from two sets to love down and break down. He’s a fighter.”
Nishikori is unbeaten in eight matches this year after winning the Brisbane International, and although Djokovic holds a 15-2 win-loss record in head-to-heads he is prepared for another gruelling battle.
“He’s a very talented player. One of the quickest players on the tour. You know, hard worker,” said Djokovic. “Yeah, I expect a tough one.”
They have met five times in Grand Slams with Nishikori’s lone win coming in the US Open 2014 semi-final.
Djokovic defeated Nishikori in the last eight of the Australian Open three years ago in straight sets and this will be their third consecutive major showdown.
The Serb won last year in the quarter-final at Wimbledon 6-3, 3-6, 6-2, 6-2 and the US Open semi-final 6-3, 6-4, 6-2 on his way to lifting the titles.
With Roger Federer already out, Djokovic is the lone player remaining with a chance to earn his seventh Norman Brookes trophy.
– Improved Raonic –
Pouille, the 28th seed who will turn 25 next month, has a former Australian Open champion in his corner as he tries to reach a first Grand Slam semi-final.
The Frenchman split with his long-time coach Emmanuel Planque in the off-season and hired Amelie Mauresmo, who won the women’s title in Australia in 2006.
He said she had helped him bring more variety to his game.
“Yeah, that’s one goal, to mix it up a lot and to be more consistent, especially from the baseline,” said Pouille, whose previous best in majors was quarter-finals at Wimbledon and the US Open in 2016.
Raonic, the 16th seed, has blasted through the tournament on the back of his supreme serving, only dropping one set — to former champion Stan Wawrinka — in the second round.
Ominously for Pouille, the Canadian former world number three thinks he has improved since he achieved his highest ranking in November 2016.
“I think I’m a better player than I was back then,” he said of developing a more aggressive serve-volley style since coming back from wrist surgery in 2017 and dropping outside the top 30.
“In the first five years of my career I probably only hit a couple thousand volleys,” added Raonic, who has won all three previous meetings with Pouille, the most recent on grass in Stuttgart last year.
“I’m sure over the last three years I’ve probably hit tens of thousands of volleys.”