Ryan Palmer out in front after second-round 67
He picked up further shots at the sixth, seventh and 11th before a solitary dropped shot at 15 left him eight under for the tournament, two shots clear.
His nearest challengers were Spaniard Sergio Garcia and Scott Piercy, who were two under for the day through seven and four holes respectively as they built on their opening 66s. Will Strickler was alone at five under after an eagle-bogey start, with six more Americans sharing fifth place at four under - Nick Watney, Joe Ogilvie and Chad Collins in the clubhouse and Brandt Jobe, Tim Herron and Zack Miller out on the front nine. Overnight leader Jeff Overton struggled to a four-over-par 74, while Dustin Johnson's challenge also faltered after two bogeys in the first six holes.