The fifth seed, who received a first round bye, was never in any real trouble against Johnson, breaking the American once in each set and facing just two break points on his own serve early in the second.
Nishikori enjoyed a breakthrough year in 2014, winning four tournaments and making the final of the US Open, and he was pleased with his start to the new campaign.
“I thought it was pretty good,??? said the Japanese star. ???I served really well, I think. I saved a couple of break points with good serves, and had a couple of aces.
“He was hitting good serves, so it was really tough to return them, but maybe I could be a little bit better with my return game.
“But it’s my first tournament, first match of the year, so I tried to play a little better as the match was going on.”
Meanwhile, fourth seed Grigor Dimitrov was given a huge scare by Frenchman Jeremy Chardy before scraping through 3-6 6-4 7-6.
Chardy was a clear frontrunner in the first set, with his booming forehand wreaking havoc, before the Bulgarian clawed his way back in the second.
He broke Chardy’s serve in the ninth game then held to level the match at a set apiece.
Games went with serve in the third, ensuring a dramatic tie-break which saw momentum swing back and forth between the two, who regularly practice together.
Chardy had two match points but couldn’t convert either, then saved two himself before Dimitrov finally clinched the match in two hours 28 minutes.
“We practice together so there are no secrets when it comes to each other’s game,” said Dimitrov said.
“He played an outstanding set-and-a-half but I stayed positive throughout the match.”
Dimitrov plays Martin Klizan in the quarter-finals after the Slovakian upset seventh seed and former finalist Alexandr Dolgopolov 1-6 7-6 7-6.
In the women’s draw, seventh seed Carla Suarez Navarro came back from losing the first set to defeat Mirjana Lucic-Baroni, while qualifying lucky loser Alla Kudryavtseva continued her good run when she beat Czech Karolina Pliskova in three sets.
Estonia’s Kaia Kanepi, the 2012 champion, beat American qualifier Madison Brengle 6-3 7-6.