American Colt Knost fired a bogey-free second round 66 at Hilton Head to move two shots clear at the halfway stage of the RBC Heritage in South Carolina.
The 26-year-old from Ohio was locked in a three-way tie for the first-round lead alongside Chad Campbell and Vaughn Taylor after an opening 67, but now leads on his own from Swede Carl Pettersson.
He made an ideal start with birdies at the first two holes before finishing a bogey-free round with five birdies overall.
Knost has had just one career top-10 on the PGA Tour, a tie for third at this year's Mayakoba Golf Classic and is 93rd on the Money List after coming through Q-School last December.
But he has so far proved the master at Hilton Head with just one bogey in 36 holes on the tight, tree-lined course.
Swede Pettersson went one better on Friday with a 65 and is a shot clear of Boo Weekley and Harris English.
Campbell dropped back into a tie for fourth after a 70, while Taylor slumped to a two-over 73.
Defending champion Brandt Snedeker shot a 67 to finish on four under in a tie for seventh, with England's Gary Christian one stroke back following his 68.
England's world number one Luke Donald bounced back from an opening 75 with a 69 to make the weekend, but at two over he is 11 shots off the pace.