Tom Carter, a three-time winner in 2003, posted his second straight 2-under 70. He stands in a tie for second place at 4-under-par 140 alongside Steven Alker, Brendon De Jonge and Jeff Freeman.
Bertsch got his second round going on the 10th tee. He parred his first six holes before stumbling to a bogey on the par-3 16th. He knocked his third shot to 5 feet to set up birdie on the par-5 18th, his ninth.
Around the turn, the 35-year-old again pitched his third within 5 feet and sank that putt for birdie. Bertsch cruised to six consecutive pars from the second at Fox Den Country Club.
Bertsch dropped a sand wedge to 3 feet at the eighth and kicked that in to get to 5-under. He parred his last hole to remain in the lead.
'Yesterday was the firmest greens I've ever played,' said Bertsch, who won earlier this year at the Charity Pro-Am at the Cliffs. 'The greens were more receptive today. I hit it well and kept it in play. You're just trying to hit greens and take your pars. It's fun playing like that.'
Carter faltered to bogeys at one and two to slip back to even-par. He came back with a 6-foot birdie putt at seven and got back to minus-2 with a 10- foot birdie putt on the ninth.
The Temple University alum moved to 3 under when he dropped in a 10-foot birdie try on the 13th. Carter ran in an 8-footer for birdie on the 16th to get within one of the lead.
'I played solid today,' said Carter. 'I played conservative and tried not to short-side myself out there. I haven't played well so far this year, but it's getting closer.'
De Jonge posted one bogey and two birdies on his opening nine to get to minus- 4. On the front side, he eagled the par-5 first, but tripped to three bogeys over a four-hole stretch from the third to slip back to 3 under. De Jonge closed his round of 71 with a birdie on the ninth.
Freeman birdied three of his first four holes to jump to minus-4. After parring the last three holes of his opening nine, the back side at Fox Den, Freeman birdied the first. He bogeyed the fourth before parring his final five holes for a 3-under 69.
Alker got to minus-4 with back-to-back birdies from the fifth. He tripped to a bogey on the seventh, but got that shot back with a birdie on No. 10. Alker bogeyed the 14th, but came back to birdie 15. He parred the last three holes to post his second straight 70.
Pat Bates, Cliff Kresge and Scott Gardiner each posted rounds of 3-under 69. They share sixth place at 3-under-par 141 with Charley Hoffman, Dave Christensen, Kim Felton and Mathew Goggin.
Brent Schwarzrock, who shared the first round lead with Jason Schultz, tripped to a 2-over 74. He is in a tie for 13th at 2-under-par 142. Schultz is one stroke further back at minus-1 at after a 75.
The cut line fell at 2-over-par 146 with 74 players advancing to the weekend.