The 37-year-old American piled up seven birdies in a flawless display for a sizzling seven-under-par 64 at the TPC Deere Run, finishing joint top with Colombia's Camilo Villegas.
Burly Zimbabwean Brendon de Jonge was a further stroke back, mixing four birdies with an eagle at the par-five 10th to end the round level with Americans Matt Bettencourt and Daniel Summerhays.
Most eyes, however, will be firmly focused on Johnson who triumphed here last year with a birdie on the second hole of a sudden-death playoff to edge out compatriot Troy Matteson.
Johnson has now posted sub-70 scores in his last 17 competitive rounds on the par-71 layout.
"I am very pleased," Johnson, a nine-times winner on the PGA Tour, told Golf Channel after making four birdies in his last nine holes. "Very few mistakes, one maybe two errant shots. I was very much in control, and very in control on the greens.
"I hit my driver beautifully, putted great and gave myself a lot of good opportunities. That's all you can ask for. Hopefully I can just continue that consistency for the remainder of the week."
Johnson was especially pleased with his driving, a component of his game which gave him trouble early on this season when he missed the cut three times in his first seven events.
"It feels tremendous," he smiled. "It's at the point now with my driver that I must have been way off (before). I feel very comfortable.
"I'm going to be hitting bad drives but I don't feel I have to grind as much off the tee box. The consistency of left-to-right and right-to-left is always nice on these tee shots.
"But I still have a lot of work to do. I am never going to be content. The leaderboard here is packed and it will be come Sunday."
One player who had a grandstand view of Johnson on Thursday was three-times former champion Steve Stricker, who was paired with his fellow American in a high-profile grouping which also included Davis Love III.
"It was a lot of fun, an enjoyable round with two friends and watching Zach play, Zach played great," said Stricker, who charged into contention at one of his favorite events with an opening 67.
"He hit in there (close) a lot of times and when he sets the pace like that, you don't want to fall too far behind. You know he's played well here in years past and you've got to stay up to him because I think he's going to be tough to beat."
Stricker made an explosive start to his round, holing out with a sand wedge to eagle his opening hole, the par-five 10th. He got to five under for the round after 11 holes before recording two bogeys over his closing stretch.
"I kept it going for a little while, then ran into a couple of bad holes at four and five on the way in," said the 46-year-old American, who was champion at the TPC Deere Run for three years in a row from 2009.
"But a good start overall. I wish I could have taken a little bit more advantage of that start but I am pretty pleased with the way I finished off the round."
South African world number 10 Louis Oosthuizen, the highest-ranked player in this week's field, was among the day's late starters.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Frank Pingue)