Sheftic, a PGA teaching pro at Merion in Ardmore, Pa., shot a 1-under 71 at Crosswater Club to match playing partner Chip Sullivan at 8-under 207.
"People normally call the third round 'Moving Day,' I call it 'Survival Day,'" the 38-year-old Sheftic said.
He had five birdies—holing a 10-footer on the par-3 17th and a 12-footer on the par-4 18th—and four bogeys.
"The pins were tough. The conditions were tough," Sheftic said. "My whole mindset today was grind it out. ... So, you tell yourself, 'Don't quit. Try to build some momentum, whether you were one back or in the lead.'"
The 48-year-old Sullivan, the 2007 winner at Sunriver, also had a 71.
"This course is playing totally different than it was in '07," Sullivan said. "It was firm and fast then, now it is wet, and balls are plugging in the fairways. The distance control has been really tricky. It gets warm on second and then it rains. I think that I put my rainsuit on eight to 10 times today. I'm worn out."
He made a 5-foot birdie putt on the par-5 16th and had 17 pars.
"I've played it (Crosswater) two times this year and have had no bogeys," said Sullivan, the PGA director of golf at Hanging Rock in Salem, Va. "I've just figured out a way to play the course.
Second-round leader Rod Perry was a stroke back after a 73. The 39-year-old Perry, the PGA head pro at Crane Lakes in Port Orange, Fla., played the first six holes in 4 over.
"I really struggled out of the gate today," Perry said. "I made a few birdies to bounce back a bit. But I am not out of it. I have a chance tomorrow. Chip and Mark are both playing extremely well and are going to be tough competitors tomorrow."
Ryan Polzin shot a 70, matching the best round of the day, to reach 6 under. The 33-year-old Polzin is the PGA head pro at Royal Oaks in Houston.
The top 20 finishers Wednesday will earn spots in the PGA Championship in August at Oak Hill in Rochester, N.Y.