Brandt Snedeker hit the front after the second round of the Safeway Open, where Phil Mickelson is three shots off the pace.
American Snedeker birdied his final two holes for a seven-under-par 65 and a three-stroke lead at Silverado Resort on Friday.
The nine-time PGA Tour champion – who won the Wyndham Championship in August – had eight birdies and a bogey to improve to 13 under at the halfway stage of the tournament in Napa, California.
"I played really solid golf all the way around," Snedeker said. "I only played two bad shots all day. My speed has been great for two days. These greens can get away from you if you're not paying attention."
Mickelson, Ryan Moore and Michael Thompson are Snedeker's nearest challengers heading into the weekend.
Ryder Cup participant Mickelson – who featured in the United States' unsuccessful campaign against Europe in Paris – followed up his first-round 65 with a 69 to be in contention.
"I'm cautiously optimistic," Mickelson said. "It's certainly turning from pessimistic because I didn't think that I was going to be here on the weekend and the next thing you know I'm up on top of the leaderboard."
First-round leader and Tour rookie Sepp Straka – making his first start – dropped down the leaderboard – the Austrian shooting a 72 to be four strokes back and tied for fifth.
Brendan Steele's bid for three successive Safeway Open titles remain alive, though he is seven under after posting a 71 on Friday.
