Northern Ireland's world number four birdied six of his final nine holes at Quail Hollow in North Carolina.

Scotsman Martin Laird, an early starter from the 10th, carded six birdies and shares second place.

World number two Justin Rose holed a 41-foot birdie putt on the opening green and returned a one-under 70.

McIlroy, who turns 30 on Saturday, struggled off the tee in the first part of his round, finding the water at the seventh to drop his only shot of the day.

But he played some magnificent shots on the back nine, including an approach from the rough to within a foot of the flag for a birdie at the 11th.

At the 493-yard par four 16th, the start of the infamous "Green Mile" closing three-hole stretch, a mammoth drive down the centre of the fairway was followed by a wedge to five feet, leading to a third successive birdie and the outright lead.

He then dispatched a drive 366 yards at the last and a wedge from 135 yards gave him another birdie chance, but the six-foot putt slid to the left.

World number 125 Dahmen joined McIlroy at the top of the leaderboard after making a birdie at the last.

The 31-year-old, yet to win on the PGA Tour, posted a bogey-free round.

Defending champion Jason Day is two shots off the lead after mixing five birdies with two dropped shots.