The in-form eighth seed felt his way into the first set against Portugal's 44th-ranked Joao Sousa before romping away to a 7-6 (8/6), 6-1, 6-2 win in 2hr 6min.
He was preceded onto Margaret Court Arena by countrywoman and US Open champion Naomi Osaka, who fought back from a set down to battle past Taiwan's Hsieh Su-wei.
The 29-year-old Nishikori has won all three of his first week matches on the same Melbourne Park second showcourt to reach the last 16 of the Australian Open for the seventh time in nine appearances.
"I love to play this court, I'm very comfortable here," said the 2014 US Open finalist after delighting a large contingent of Japanese fans.
"It's great to play here, especially after Naomi today."
Nishikori and Sousa had each survived two five-set epics before their third-round clash.
It looked odds-on for another marathon when they went shot-for-shot in the first set which Nishikori edged in a tiebreak after almost an hour.
But as the temperature began to rise in the mid-afternoon sunshine, he clearly decided he didn't want to go the distance again.
He stepped hard on the accelerator, powering 19 unerring winners on his deadly forehand as he took just over an hour more to race through the second and third sets.
"The forehand is always my favourite shot," said Nishikori who won the Brisbane International warm-up event and has extended his 2019 unbeaten run to seven matches.
"It's not just the forehand, everything was working well today," added Nishikori, who will play either 23rd seed Pablo Carreno Busta of Spain or Italy's Fabio Fognini, seeded 12, for a place in the quarter-finals.