William McGirt shot 65 to lead on 12 under par after three rounds of the Northern Trust Open in California.
The 34-year-old, looking for a first PGA Tour win, made eight birdies to take a two-shot lead into the final round at the Riviera Country Club.
American compatriots George McNeill and Charlie Beljan are also in contention after rounds of 66 and 68 respectively.
Englishman Lee Westwood hit a much-improved three-under-par round of 68 but is still off the pace at six under.
McGirt began two strokes off the lead when the delayed second round was completed early on Saturday, but took advantage of several favourable pin positions with some superb all-round golf.
He birdied five of his first six holes, then picked up three more shots in the first four holes after the turn.
"I played really solid, hit a lot of good iron shots and good putts," said McGirt. "It was Just one of those days you could see the line."
McNeill eagled the par-five finish to finish 10 under and level with Beljan while Jason Allred and Brian Harman are a further shot behind.
South Korean Bae Sang-Moon, who led after the second round, is among a group including Bubba Watson and Dustin Johnson at eight under.
Justin Rose, who hit 68, is three under but Ian Poulter could do little to improve his standing after he shot 71 to finish level par overall.