American Austin Cook carded a round of 64 to jump nine places up the leaderboard and take a one shot lead into the final day of the CareerBuilder Challenge in California.
Waterford's Seamus Power is just four shots behind after shooting a 65.
Cook, 26, was already six under at the turn, having birdied the 4th, 5th, 7th and 9th and eagled the par-five 6th.
A wobble on the 10th was followed by birdies on the 11th and 12th, and a solid tee shot at the par-three 15th wound up five feet from the hole to get a birdie and seal a slender lead over the chasing pack.
Andrew Landry, the leader at the halfway point, carded a 70 to finish 18-under along with Martin Piller, who followed Friday's 67 with the same score on the LaQuinta course.
Two players - Spain's Jon Rahm and American Scott Piercy - sit tied for fourth on 17-under.
Four birdies and two bogeys were all defending champion Hudson Swafford could manage as he sits tied for 21st on 12 under, along with the highest placed Briton Russell Knox, who had an up and down round of 71.
Power shot seven birdies, with a bogey on 17 the only negative on a good day as he lies in a tie for ninth place.