Coston, a 48-year-old teaching pro at Semiahmoo Golf Resort in Blaine, Wash., had a course-record 64 Friday and is at 12-under 204 at Longaberger Golf Club.
'Maybe if I won the U.S. Open or the PGA Championship, my life would change,' Coston said. 'But even if I win this I'll still be back at work next week.'
Bob Sowards of suburban Columbus matched the low round of the day with a 69 and was two back at 206.
'My goal was four rounds in the 60s,' Sowards said. 'I figured I'd be in good shape. I've got three of them. Now I need one more.'
Looking toward the final round, Coston said he wouldn't get caught up in what his closest pursuer and playing partner was doing.
'I can't worry about Bob,' Coston said. 'I've just got to worry about Uncle Jeff.'
University of Illinois men's golf coach Mike Small shot a 70 and was at 208. Small was the first-round leader after a 66, then fell four shots behind Coston with a second-round 72.
Robert Thompson Jr. of Huntsville, Texas, shot a 72 and was at 209, followed by Chip Sullivan of Roanoke, Va., at 210 after a 71.
The top 25 finishers earn spots in the PGA Championship next month at Whistling Straits in Kohler, Wis. The winner collects $60,000.
Coston birdied the first hole and led by as many as five shots before Sowards birdied Nos. 15 and 16 to cut the lead to two strokes.
'My goal this week, as much as anything, was to have fun and not grind too much,' said Coston, who has missed the cut in half his six previous Club Pros and has never finished higher than a tie for 27th. 'I didn't get these wrinkles and gray hair from watching golf on TV.'