Jason Dufner remained on course for a second straight tournament win with a two-stroke lead at the halfway point of the Crowne Plaza invitational at Colonial, Texas.
Dufner, also bidding for a third win in five weeks following his success at last week's Byron Nelson Championship, fired a bogey-free six-under round of 64 to lead by two shots.
Dufner had an 11-under 129 total at Hogan's Alley, with first round leader Zach Johnson slipping to second after a 67.
Bo Van Pelt and Tommy Gainey were third at seven under after Van Pelt shot a 64 and Gainey a 67.
Johnson had his 14th consecutive sub-par round at Colonial, one more than Van Pelt but defending champion David Toms missed the cut after shooting rounds of 74 and 71.
Dufner admitted he aimed to continue to show the good form which had put him in place to secure another win.
Dufner told PGA Radio: "My mindset is just to continue to play well and to be confident with it.
"If I play some bad shots I just have to realise that I'm playing good golf and continue to run with it."
Dufner revealed slightly stronger winds had made the second round tricky.
"I had a really nice day again today. The winds were up a little bit this morning which made it a little bit more difficult.
"I had another solid day. I kept the ball in play pretty well. I made some putts and all in all I had another great day.
"It's been a good stretch of golf and I look forward to playing the weekend."
In fifth is Tom Gillis, with fellow Americans Ryan Palmer and Harris English tied sixth and Rickie Fowler, John Huh and Ryan Moore sharing eighth.
England's Greg Owen is in a tie for 12th but Sergio Garcia lies joint 21st.
Collated second-round scores (Par 70):
129 Jason Dufner (USA) 65 64
131 Zach Johnson (USA) 64 67
133 Bo Van Pelt (USA) 69 64, Tommy Gainey (USA) 66 67
134 Tom Gillis (USA) 65 69
135 Ryan Palmer (USA) 68 67, Harris English (USA) 65 70
136 Bryce Molder (USA) 72 64, Ryan Moore (USA) 67 69, John Huh (Kor) 70 66, Rickie Fowler (USA) 68 68
137 Charlie Wi (Kor) 68 69, Kyle Reifers (USA) 65 72, Andres Romero (Arg) 66 71, Greg Owen (Eng) 69 68, Kevin Chappell (USA) 70 67
138 Jim Furyk (USA) 69 69, Bobby Gates (USA) 71 67, David Mathis (USA) 71 67, Louis Oosthuizen (Rsa) 71 67
139 Seung-yul Noh (Kor) 70 69, Charley Hoffman (USA) 69 70, Sung Kang (Kor) 70 69, Sergio Garcia (Spa) 66 73, Greg Chalmers (Aus) 70 69, John Daly (USA) 70 69, Vijay Singh (Fij) 70 69, Carl Pettersson (Swe) 70 69, Tim Clark (Rsa) 70 69
140 Chris Kirk (USA) 71 69, Chris Di Marco (USA) 66 74, John Senden (Aus) 69 71, Gary Woodland (USA) 70 70, Geoff Ogilvy (Aus) 70 70, Jonas Blixt (Swe) 73 67, Hunter Mahan (USA) 69 71, Michael Thompson (USA) 69 71, Marc Leishman (Aus) 72 68, Sang-moon Bae (Kor) 70 70, Jonathan Byrd (USA) 72 68, Jason Bohn (USA) 70 70
141 Brendon De Jonge (USA) 67 74, Martin Flores (USA) 68 73, Will Claxton (USA) 72 69, Matt Kuchar (USA) 72 69, William McGirt (USA) 70 71, Blake Adams (USA) 69 72, Kevin Na (USA) 70 71, Aaron Baddeley (Aus) 71 70, George McNeill (USA) 72 69, Ben Crane (USA) 70 71, Trevor Immelman (Rsa) 70 71, Brendon Todd (USA) 70 71, Corey Pavin (USA) 71 70, Roberto Castro (USA) 67 74, Josh Teater (USA) 70 71
142 Nick Watney (USA) 71 71, Jerry Kelly (USA) 72 70, Chris Stroud (USA) 69 73, Rory Sabbatini (Rsa) 71 71, Kris Blanks (USA) 73 69, Kelly Kraft (USA) 71 71, Mark Wilson (USA) 71 71, J.J Killeen (USA) 72 70, Justin Leonard (USA) 69 73, Chad Campbell (USA) 71 71, Brandt Jobe (USA) 67 75, Y.E. Yang (Kor) 73 69
143 Pat Perez (USA) 69 74, David Hearn (Can) 71 72, Gary Christian (Eng) 70 73, John Mallinger (USA) 71 72, Heath Slocum (USA) 70 73, Edward Loar (USA) 72 71, Bill Haas (USA) 72 71, Miguel Angel Carballo (Arg) 75 68, Boo Weekley (USA) 72 71, Ken Duke (USA) 68 75, Hunter Haas (USA) 71 72
------------------------The following players failed to make the cut-------------------
144 Bud Cauley (USA) 71 73, Spencer Levin (USA) 72 72, Jeff Overton (USA) 71 73, Vaughn Taylor (USA) 74 70, Graham Delaet (Can) 72 72, Ryo Ishikawa (Jpn) 74 70, Jeff Maggert (USA) 72 72, J J Henry (USA) 71 73, John Rollins (USA) 72 72
145 Ricky Barnes (USA) 74 71, Bob Estes (USA) 72 73, Rodney Pampling (Aus) 73 72, Tim Herron (USA) 72 73, David Toms (USA) 74 71, Daniel Summerhays (USA) 67 78
146 Kyle Stanley (USA) 72 74, Michael Bradley (USA) 75 71, Brian Gay (USA) 74 72, Davis Love III (USA) 75 71, Billy Mayfair (USA) 72 74, Harrison Frazar (USA) 72 74, James Driscoll (USA) 72 74, Kevin Streelman (USA) 73 73, Kevin Stadler (USA) 74 72, Johnson Wagner (USA) 76 70, Cameron Tringale (USA) 75 71, D.J. Trahan (USA) 70 76
147 Fulton Allem (Rsa) 73 74, Scott Stallings (USA) 72 75, Matthew Every (USA) 73 74, Mark Anderson (USA) 75 72, Chez Reavie (USA) 73 74
148 Billy Hurley III (USA) 71 77, Steve Flesch (USA) 73 75, Brian Harman (USA) 76 72, Henrik Stenson (Swe) 72 76
149 Dicky Pride (USA) 76 73
150 Erik Compton (USA) 72 78, Colt Knost (USA) 74 76, Lucas Glover (USA) 74 76
154 Peter Uihlein (USA) 74 80, Robert Allenby (Aus) 75 79
159 Keith Clearwater (USA) 78 81
