Roger Federer extended his 2018 win streak to 13 matches and has only dropped three sets from 34 played this year.
Former world number one Novak Djokovic's toughest hurdle when he returns to the ATP Tour after a long injury absence will be a mental one, believes former coach Boris Becker.
Novak Djokovic did not reveal when he planned to return to the tour.
Former World No1 Novak Djokovic lost to Chung Hyeong in straight sets in the fourth round.
Roger Federer and Novak Djokovic kept up their relentless march towards a semi-final meeting at the Australian Open Saturday
Swiss 19-time Grand Slam champion Federer disposed of Jan-Lennard Struff 6-4, 6-4, 7-6 (7/4), avoiding the worse of the furnace-like conditions in his night match.
Djokovic said there was little indication that players were going to have any form of discussion with the sport's chiefs for a shorter season.
Djokovic plays Spain's 21st seed Albert Ramos-Vinolas in the third round.
The 12-times Grand Slam champion, Djokovic, seeded an unfamiliar 14, was too strong for American Donald Young, winning 6-1, 6-2, 6-4.
Rafael Nadal, who is gunning for a 17th major title but only his second Australian Open crown, was hampered by a knee injury at the tail-end of the 2017 season.
Djokovic made a winning return on Wednesday after six months out with an elbow injury, declaring he felt "great" after beating world number five Dominic Thiem.
Djokovic revealed he felt "great" after beating world number five Dominic Thiem 6-1, 6-4 at the Kooyong Classic.
World number one Nadal, who missed the warm-up Brisbane International last week with a knee injury, is set for just one appearance on Tuesday when he faces France's Richard Gasquet.
Asian number one Nishikori has not played competitively since last August after suffering a torn tendon in his right wrist during a practice session at Cincinnati.
His withdrawal comes just 24 hours after the injury forced him to drop out of an exhibition tournament in Abu Dhabi.
Novak Djokovic, who has been sidelined since July with a right elbow injury, is making his long-awaited return to the game at an exhibition tournament in Abu Dhabi from December 28-30
Former world number one Novak Djokovic said Radek Stepanek was joining his coaching team to complement Andre Agassi.
Murray is due to face world number two Roger Federer in a charity exhibition match in Glasgow on Tuesday before he starts his preparations for the Australian Open by playing in Brisbane.