Rory McIlroy ended day two of the Wells Fargo Championship five shots off the lead after a difficult final two holes in North Carolina.
The Northern Irishman started Friday with a share of top spot following an opening round of 66, and looked set to improve his standing when he moved to nine under, having carded five birdies and one bogey after 16 holes.
But four-time major winner McIlroy, who turns 30 on Saturday, slipped back toward the chasing pack as he recorded a double-bogey on the penultimate hole, before missing the green and bogeying the last.
McIlroy is a two-time winner of this event, but his one-under-par 70 dropped him back to six under overall, which is five shots behind clubhouse leader Jason Dufner.
Dufner, 42, had a tremendous day at Quail Hollow as he surged past McIlroy with one of the best performances of the round.
He shot a 63 and sank a nice putt from 40-feet during his impressive showing, to become the sole leader at 11 under in the tournament.
Dufner has missed more weekends than he has played this season and is interested to see what he can do heading into round three.
"We'll see how it goes being in the heat of it on Saturday and Sunday," he said.
"I've been there before. It's been a while, but I kind of know what to expect. It will be a good challenge to see where I'm at, what I'm doing."
Joel Dahmen and Max Homa sit one shot off Dufner at 10 under, with the latter matching Dufner's round of eight under on Friday.
Patrick Reed is tied with McIlroy at six under and they round out the top five. Phil Mickelson missed the cut after shooting five over for the tournament.