Hjertstedt, the 1997 B.C. Open winner, is in position to challenge for his third career title and first since winning the 1999 Tucson Open.
A co-second-round leader, he stands at 14-under-par 202 and holds the first 54-hole lead of his career despite stumbling to bogeys at the 16th and 17th holes.
'Anytime you are in front, that's a good thing,' said Hjertstedt, who currently ranks 189th on the PGA TOUR money list and 65th on the Nationwide Tour's.
Mark Brooks, the 1996 PGA champion and first-round leader, matched the round of the day with a 6-under 66 and shares second place with David Branshaw (68) and co-second round leader Scott Gump (69) at 13-under-par 203.
The third overnight co-leader, rookie Daisuke Maruyama of Japan, managed just a 1-under 71. He is tied for fifth place at minus-11 with 1987 Masters champion Larry Mize (67), Matt Gogel (67) and John Rollins (68).
Hjertstedt, 34, began the round tied for the lead with Gump and Maruyama at 10-under-par. His birdie at the par-4 first -- combined with bogeys from Gump and Maruyama -- gave him a two-shot lead.
Another birdie at the fourth and two straight at the seventh and eighth put Hjertstedt at 14-under for a three-shot lead. He continued to cruise after that, rolling in a 4-foot putt at the 12th and a 14-footer at the 14th for two more birdies.
Hjertstedt's birdie putt at the par-4 14th rested at the edge of the cup for several seconds and then dropped in. His premature putter-pump celebration devolved awkwardly into a putter twirl before the ball fell in.
'The greens are just so phenomenal here that if you are rolling the ball well, you'll be in it,' he said.
Hjertstedt went on to make a mess of the 16th hole, where he missed a 12-foot par putt for his first bogey of the round, and the 17th, where he made another bogey from a greenside bunker.
'If you look at the good things from today, I struck the ball really nice,' said Hjertstedt. 'I had a couple of holes there coming in where I was in between clubs. I just didn't seem to get them right.'
Brooks eagled the par-4 fourth and posted back-to-back birdies twice during his third round, tying for the day's best score with Paul Stankowski. His second set of consecutive birdies came at the 16th and 17th holes.
'I just went out and tried to play solid,' said Brooks. 'It's in contention right now, but you have five more hours of golf left. Who knows who will be leading by the end of the day.'
Branshaw, who was alone in fourth place overnight, bogeyed his first hole, then played flawless golf the rest of the way. He birdied two of his final four holes to move into a share of second place.
'It played difficult out there today. It played much longer,' Branshaw contested. 'The greens were a lot softer, but there were more footprints out there because of it.'
Like Branshaw, Gump bogeyed No. 1 and then played the rest of the round without a dropped shot. He made all four of his Saturday birdies over the last seven holes at Turning Stone Resort's Atunyote Golf Club.
Of course, that took a long time. The round was suspended twice due to inclement weather.
'Wow, that was a long round,' said Branshaw. 'I teed off at 9:15. It was just your normal 9-, 10-hour round.'
Shigeki Maruyama, Bob May and Daniel Chopra -- who all shared 27th place overnight -- moved into a tie for ninth at 10-under-par 206 following rounds of 68.
Scott Gutschewski, Shane Bertsch, Frank Lickliter II and Mathias Gronberg are also at minus-10.