Winner of the PGA Tour event in 2012, the 38-year-old fired an eight-under-par 63 in ideal scoring conditions at the TPC Deere Run to end the round level with fellow American Brian Harman and South African Rory Sabbatini.
Johnson, with his driving and renowned short game in sparkling order, covered his outward nine in an explosive six-under 30 to raise thoughts of a possible 59 but he picked up just two more shots after the turn.
American Todd Hamilton, surprise winner of the 2004 British Open held at Royal Troon, and Australian Steve Bowditch opened with 64s while Zimbabwe's Brendon de Jonge and American Kevin Tway carded 65s.
Johnson, who was born just 90 miles away in Cedar Rapids, Iowa and has recorded top-three finishes in four of his last five starts in his "hometown" event, was delighted with his opening round.
"Extremely pleased," the 11-times PGA Tour winner told Golf Channel. "The word of the day for (his caddie) Lance (Bennett) and me was opportunity and I had a lot of opportunities.
"I missed just three greens but I putted on two of them. So I put myself into position to make birdies, I put myself into position basically to eliminate the big numbers, certainly even a bogey.
"Everything was really solid," said Johnson. "I drove it great, a couple of iron shots I'd like to have back but nothing significant. I actually had a couple more putts that could have dropped but I'm not complaining."
Asked whether he had thoughts of shooting a 59 after getting to eight under after 11 holes, Johnson replied: "It hit me. I'm like, 'How many holes have we played, how many under am I?' That kind of thing.
"But over shots and in the routine of shots - nothing. I hit a lot of really, really good shots that ended up probably 20, 25 feet away but this course, you've got to respect it.
"There are some (pin) placements out there where it's hard to be aggressive at. I had some (birdie) looks certainly on eight and five that came up a little bit curvey. But, for the most part, it was really solid."
American Jordan Spieth, who won last year's John Deere Classic in his rookie season to become at 19 the youngest player to triumph on the PGA Tour since 1931, was among the late starters on Thursday. - Reuters
source: gmanetwork.com
