Johnson shot a 67 following by a 69 to end-up four-under-par at the halfway point, after having to play the course twice following rain delays.

"I'm in pretty good shape, so the physical part's no problem," he said after his marathon day.

"But mentally you've got to make sure you stay sharp all day, because you can't go to sleep on any shot out here.

"I hit so many good putts today that I thought were going in, and burned the edge or lipped out.

"That's just how it goes, these greens are tough."

Johnson had a putt to win last year's US Open but took three to hand the title to fellow countryman Jordan Spieth.

Round one leader Andrew Landry still has to start his second round, but is also on four under.

Spaniard Sergio Garcia posted a second round of 70 to stay on two-under, alongside another American in Scott Piercey.

England's Lee Westwood completed an impressive first round of 67 but is also yet to start his second.

Others still one round behind are defending champion Spieth, who shot a first round of 72, England's Masters champion Danny Willett and Northern Ireland's Rory McIlroy.

Both need to improve to make the cut after poor first rounds of 75 and 77 respectively.