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.
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.
Collated second round scores & totals in the USPGA Tour RBC Heritage, Harbour Town Golf Links, Hilton Head Island, South Carolina, United States of America
(USA unless stated, par 71):
133 Colt Knost 67 66
135 Carl Pettersson (Swe) 70 65
136 Boo Weekley 70 66, Harris English 68 68
137 Chad Campbell 67 70, Robert Garrigus 71 66
138 Kevin Na 70 68, Bob Estes 71 67, Fredrik Jacobson (Swe) 71 67, Michael Bradley 74 64, Brandt Snedeker 71 67
139 Bud Cauley 71 68, Charley Hoffman 74 65, Zach Johnson 71 68, Gary Christian (Eng) 71 68
140 Brian Davis (Eng) 72 68, Vaughn Taylor 67 73, Greg Chalmers (Aus) 71 69, Charles Howell III 72 68, Chez Reavie 69 71, Matthew Every 68 72, Tim Clark (Rsa) 73 67, Tommy Gainey 70 70
141 Jerry Kelly 72 69, Lee Janzen 71 70, Brian Harman 71 70, Geoff Ogilvy (Aus) 74 67, Matt Kuchar 72 69, John Mallinger 69 72, Charlie Wi (Kor) 68 73, Joe Durant 70 71, Michael Thompson 71 70, Tom Gillis 70 71, Jason Bohn 70 71
142 Brendon De Jonge 72 70, Rory Sabbatini (Rsa) 70 72, Billy Mayfair 72 70, Will Claxton 70 72, Heath Slocum 71 71, Graham Delaet (Can) 74 68, Kevin Chappell 70 72, Matt Bettencourt 73 69, Kevin Streelman 74 68, D.A. Points 74 68, J J Henry 72 70, John Rollins 70 72, Trevor Immelman (Rsa) 71 71, Sean O'Hair 73 69, Marc Leishman (Aus) 71 71, Stuart Appleby (Aus) 73 69
143 Kevin Stadler 72 71, Jim Furyk 68 75, Kyle Stanley 71 72, Briny Baird 74 69, Nick O'Hern (Aus) 74 69
144 Jason Dufner 78 66, Lucas Glover 72 72, James Driscoll 72 72, Mark Anderson 73 71, John Daly 70 74, Cameron Beckman 73 71, Ken Duke 72 72, Luke Donald (Eng) 75 69, Rocco Mediate 73 71
145 Spencer Levin 73 72, Jeff Maggert 74 71, Shaun Micheel 72 73, Hunter Haas 71 74, Mark Wilson 73 72, Stephen Ames (Can) 71 74, Webb Simpson 71 74
Missed the cut:
146 Chris Kirk 76 70, Corbin Mills 72 74, Kris Blanks 74 72, Brian Gay 75 71, David Hearn (Can) 78 68, Chris Couch 71 75, William McGirt 74 72, Tom Pernice Jnr. 75 71, Blake Adams 69 77, Ricky Barnes 74 72, Glen Day 70 76, Henrik Stenson (Swe) 74 72, Padraig Harrington (Irl) 74 72, Tommy Biershenk 76 70, D.J. Trahan 74 72
147 Bryce Molder 73 74, Tim Herron 75 72, David Duval 76 71, Scott Piercy 74 73, Chad Collins 73 74, Arjun Atwal (Ind) 76 71, Rich Beem 77 70, Daniel Summerhays 73 74
148 Jeff Overton 73 75, John Merrick 74 74, Greg Owen (Eng) 74 74, J.J Killeen 72 76, Bill Haas 74 74, Ernie Els (Rsa) 72 76, Martin Laird (Sco) 72 76, Scott Verplank 73 75, Troy Matteson 72 76
149 Aaron Baddeley (Aus) 73 76, Hank Kuehne 73 76, Martin Flores 75 74, Bill Lunde 74 75, David Mathis 74 75, John Huh (Kor) 77 72
150 Jose-Maria Olazabal (Spa) 77 73, Robert Karlsson (Swe) 72 78, Justin Leonard 78 72, Jason Kokrak 73 77, Chris Di Marco 75 75, Bo Van Pelt 74 76, Rickie Fowler 72 78
151 Sung-hoon Kang (Kor) 75 76, George McNeill 75 76
152 Robert Allenby (Aus) 78 74, Joe Ogilvie 74 78
153 Ryuji Imada (Jpn) 76 77, Fred Funk 81 72
154 Seung-yul Noh (Kor) 76 78, Scott Stallings 80 74, Josh Teater 78 76
156 Brendon Todd 76 80, Stewart Cink 79 77
157 Mike Weir (Can) 76 81
159 Todd Camplin 77 82
WD: 72 Alex Cejka (Ger), 79 Camilo Villegas (Col)
