Todd Gleaton and Joe Ogilvie, a two-time winner on tour in 2003 and second on the money list, posted rounds of 5-under-par 66 and share fourth place.
Dufner wasted little time in breaking into red figures Thursday although he did par No. 1. He knocked a seven-iron to 12 feet to set up birdie at the second, holed a two-footer for birdie at the third and capped off his run of birdies with a 10-foot birdie at No. 4.
Dufner, who won the Wichita Open in 2001, rolled home a six-footer for birdie at the seventh to make the turn at 4-under-par 31.
At the 10th, Dufner knocked a nine-iron to six feet to set up birdie. He parred the next four holes but found trouble at the 15th when his sand-wedge approach sailed over the green and he failed to save par.
Dufner rebounded on the closing holes with a 20-foot birdie at 16 and a six- footer for birdie at the par-five finishing hole to match his lowest round on the Nationwide Tour in 2003.
'I played pretty good. Seven birdies is good and 6-under is not too bad,' said Dufner. 'You've got to make putts. Since '01, I haven't had too many good putting rounds but today I made some putts.'
Bertsch opened with back-to-back birdies at one and two, from 15 and eight feet, respectively. He added another pair of birdies at seven and eight to head to the back nine at minus-4.
For the third time in Thursday's round, Bertsch collected back-to-back birdies. He sank a 30-footer at the 10th and an eight-footer at the 11th but three-putted from 30 feet for bogey at No. 12.
Bertsch left a few strokes at Empire Lakes on the back nine. He missed birdie putts from five, nine and four feet at holes 13-15 but joined the group at the top of the leaderboard with a two-putt birdie at the last.
'That could've been one of those 8- or 9-under rounds,' admitted Bertsch, who tied for eighth place at the beginning of the month in Utah. 'I left a few out there on the back but good things happened too.'
Oh, who started on the back nine Thursday, tallied three birdies on each of his nines to card his bogey-free 65.
Casey Martin, Doug LaBelle II, Hunter Haas, John Elliott, Jason Bohn, Ryan Howison, Robert Conrad, Kyle Kovacs, Michael Allen and Brad Ott are tied for sixth place at 4-under-par 67.