The 31-year-old, who came agonisingly close to winning the US Open last year, then made a further four birdies on the back nine to stake an early clubhouse lead.

Despite the efforts of compatriot Brendan Steele, who is a shot further back on seven under par, Johnson maintained that advantage and heads into the second day at Muirfield Village in good spirits.

"With me, it has everything to do with the putter," Johnson said on www.pgatour.com. "I rolled it well today. I've been working pretty hard on the putter, and I felt like it's finally starting to pay off."

World number one Jason Day is among a host of players on six under par, including 2013 winner Matt Kuchar, while Luke Donald is the best-placed Briton on five under.

Two weeks ago, Rory McIlroy won the Irish Open.

This week he made a major putting change. https://t.co/lyXcYFj1UL pic.twitter.com/oEOpjEqDpz

— PGA TOUR (@PGATOUR) June 3, 2016

Rory McIlroy, who won his first title of the year at the Irish Open two weeks ago, carded a round of 71 and is seven shots off the lead.

The Holywood golfer, though, drew attention for his change to a conventional putting grip having won using the cross-handed method in Ireland.

McIlroy said: "I'm using the line on the ball again and, sometimes even on the course, it feels uncomfortable because sometimes I don't feel like the line is aiming where it actually is.

"So I need to work on the ball position and my eye position. So I'm trying to be really structured with that. I feel like, if I'm consistent with that, that will give me the best chance going forward."

World number two Jordan Spieth - fresh from his Colonial Invitational victory - is well-placed at two under par along with England's Paul Casey.