World number six Jordan Spieth had stood one shot off the pace after an opening round of 66, and still stood at six-under through six holes on Friday, and Lingmerth was six-under through eight.
Dufner, who began his second round on the 10th hole, followed the startling eagle with a string of three birdies in a row from the fifth.
But his first bogey of the day on the par-four last saw him post a seven-under 65.
"I caught some breaks when I missed the fairway and still had some fairly good opportunities from the rough", Lingmerth said.
"I guess [14 under] was out there", Fowler said.
Harrington carded a one-under par round of 71 to lie six shots off the lead, held jointly by Jason Dufner and David Lingmerth, whil Lowry signed for a level par 72, which included three birdies and three bogeys. "There's a lot of work ahead this week and weeks to come obviously so I'm just trying to take one shot at a time".
He rebounded, however, with five birdies on his inward run.
Meanwhile Johnson, who will defend his maiden major title at the US Open at Erin Hills in Wisconsin in two weeks, got off to a dismal start in this crucial tune-up.
Top-ranked Dustin Johnson, who struggled to a 78 in the first round, was still six-over through six holes of his second round and in danger of missing the cut, projected to be at two over par. The world's No. 1 player three-putted the par-3 16th after hitting into the water and made triple bogey.