Lucas Glover and Ted Purdy each posted 67s on Friday and share third place at 8-under-par 136. Bo Van Pelt shot a 4-under 68 in the second round and is alone in fifth place at minus-6.
Johnson, a Monday qualifier with conditional status on the Nationwide Tour, continued his momentum from Thursday as he was in the early wave of tee times on Friday. He missed a four-foot birdie putt at his first hole, the 10th, but rebounded with a 10-footer for birdie at No. 11.
'I was glad to be teeing off early today,' said Johnson. 'You like to keep the momentum going into the next day.'
The Duke University graduate holed a right-to-left 20-footer for birdie at 15 and followed with a 15-foot birdie at the very next hole. Johnson closed his first nine with a six-footer for birdie at the par-5 18th.
Johnson collected birdies at three, four and six to reach 14-under par. He dropped a stroke to par after he missed an eight-footer at the eighth but responded with a beautiful 7-iron to 15 feet at the ninth which set up birdie and gave him a four-shot lead.
'As far as difficulty of the golf course combined with the way I'm playing, it's probably one of my best starts ever,' he said. 'I made a ton of putts. Even the ones I missed were solid. It was pretty ridiculous but I missed a couple too, so it all evens out. If I can get it on the right quadrant of these greens, I feel like I have a chance to make it.'
Johnson found himself in a similar spot two years ago when he held a one-shot lead through 36 holes of the Tri-Cities Open. Tough winds and a difficult course meant weekend scores of 78-75 and a tie for 25th.
'There is a lot of golf left, for one thing,' said Johnson. 'I have to try and stay within myself and not get caught up in what others are doing. If I get caught up in numbers, it can go backwards pretty quick.'
Van der Walt was flawless on Friday with six birdies and no bogeys. He holed out from a bunker for birdie and made a double sand save at his closing hole, the ninth, when he drove into a fairway bunker and found a trap near the green.
'Iron play is the key out here,' said Van der Walt. 'You're hitting a lot of long irons. You're coming in with 4-, 5-, 6-iron and the greens are hard as a table.'
Joe Ogilvie (65), Scott Petersen (70), Chris Tidland (70), Mike Springer (72), Danny Ellis (67), Boo Weekley (69), Steve Haskins (69), Brian Henninger (69) and James Driscoll (70) share sixth place at 5-under-par 139.
The 36-hole cut fell at 1-under-par 143 and 61 players advanced to the weekend. Among the notable players who missed the cut were Casey Martin (145) and former PGA Tour players Mike Hulbert (147) and Chip Beck (148).