Scott Petersen, Travis Perkins, Andrew Buckle and Steve Larick are knotted in fourth place at 5-under-par 67 at the Tournament Players Club at Virginia Beach.
Hjertstedt, a former member of the PGA TOUR, flew out of the gate on Thursday with back-to-back, 15-foot birdie putts at one and two. He rolled in a 12-foot birdie putt at the fifth, then got to minus-4 with a 25-footer at the seventh.
The Swede reached the par-5 eighth green in two with a 4-iron, then sank the 40-footer for eagle. Hjertstedt converted a 12-foot birdie putt at nine to go out in an eye-opening, 7-under 29.
'I just hit every shot perfect on the front nine,' said Hjertstedt. 'I didn't miss much. It's always nice to get it rolling like I did on the front side today.'
Hjertstedt did not get anything going on the back nine. He fell back to minus-6 with a bogey at the 203-yard, par-3 13th, but reclaimed the lost stroke with a 10-foot birdie putt at 14.
He was alone in the lead at 7 under par, but a bogey at the par-3 17th dropped Hjertstedt into a share of the lead.
'It was just a solid day all the way around,' said Hjertstedt. 'I didn't pull away on the back because I hit a couple of stray ones, but I've got nothing to complain about.'
Pratt sank a 10-foot birdie putt at the fourth and added a 4-foot birdie putt at the eighth to make the turn at 2-under-par 34. He moved up the leaderboard with his play early on the back nine.
He birdied No. 10, then kicked in a short birdie putt at the 12th. Pratt made it two in a row thanks to a 30-foot birdie putt at the 13th. He rolled in a 20-footer for birdie at No. 15 to get to minus-6.
'I played really solidly today,' said Pratt, who has not played on the Nationwide Tour this season, but rather the Hooters Tour. 'I'm hitting a lot of fairways and greens and giving myself some good putts.'
Merrick collected his first birdie of the round from 15 feet at the second hole. He tapped in a short putt for birdie at the fifth, than rattled off three birdies in a row from the seventh.
Merrick, who tied for 10th two weeks ago at the Athens Regional Foundation Classic, holed a 3-foot birdie putt at the 11th. He drained a long bomb for birdie at 13 to make it to 7 under par, but a missed 10-footer at the 14th resulted in a bogey.
'I'm really pleased,' said Merrick. 'My game is really good. I hit the ball great and made some putts.'
Tripp Isenhour, a two-time winner this season on the Nationwide Tour, Michael Long, Brendon de Jonge, David Hearn, Chris Nallen, Joel Edwards, Cliff Kresge, Paul Gow, Gary Christian and Hiroshi Matsuo share eighth place at 4-under-par 68.