Tyler Duncan took control of the Safeway Open after moving into the outright lead at the halfway stage of the tournament as Chesson Hadley shot a course record.
Duncan was in a three-way tie with defending champion Brendan Steele and Tom Hoge atop the leaderboard when the second round teed off on Friday.
But the 28-year-old American, yet to win on the PGA Tour, earned a one-stroke lead at the Silverado Country Club after finishing day two birdie-birdie-eagle.
Duncan carded a six-under-par 66 - including a 45-foot chip-in at the par-five ninth - to be 13 under through 36 holes heading into the weekend in Napa, California.
Steele dropped to second in his bid for back-to-back titles following his second-round 67.
A two-time Tour champion, Steele had six birdies to go with his solitary bogey at the 11th hole.
Fellow American Hadley is two shots off the pace following his record round after flirting with a 59.
10 birdies.-- PGA TOUR (@PGATOUR) October 7, 2017
Hadley posted an 11-under-par 61 to move from a tie for 61st to solo third.
The 30-year-old needed an eagle at the 18th to shoot 59 but parred for a 61.
Zac Blair (66) and Tony Finau (65) are two shots further back, while Graham DeLaet (67) and Brian Davis (66) are eight under overall.
Five-time major winner Phil Mickelson recorded a 69 to be six under, seven shots behind Duncan.