Maverick McNealy moved to the top of the Fortinet Championship leaderboard after an up-and-down round on Friday in California.
McNealy shot an eight-under-par 64 to take a two-stroke lead on 12-under after two rounds.
Beau Hossler also carded a 64 at Silverado Resort and Spa's North Course in Napa Valley, and he shares second place with Chile's Mito Pereira, who produced a 67. Both are on 10 under.
Will Zalatoris, Bronson Burgoon and Troy Merritt are tied for fourth place on nine under.
Matt Kuchar, Scotland's Russell Knox and Harold Varner III are on eight under, tied for seventh.
World number one Jon Rahm can focus on preparing for the Ryder Cup next week after missing the cut. The Spaniard finished on one under to miss the cut by two strokes.
Reigning Masters champion Hideki Matsuyama of Japan logged his second consecutive 69 and is tied for 17th on six under.
Matsuyama is one shot ahead of Phil Mickelson, who shot a 69 on Friday and is tied for 24th.
McNealy, 25, is still in search of his first PGA Tour win in four years since turning pro after a stellar collegiate career at Stanford. His best finish came earlier this year at another Northern California event, the AT&T Pebble Beach Pro-Am.
Starting on the back nine, McNealy birdied three of the first four holes and was five under on the day at the turn.
He then bogeyed 10, 11 and 12 before following immediately with four consecutive birdies, capped by holing out from a greenside bunker. Following a par at the eighth, he closed by chipping in for eagle on the ninth hole.
"It was a rollercoaster day," McNealy said. "The (first) nine I played just about flawless golf, hit it where I wanted to. Then I got to the first hole and funny lie in the right rough and dumped it in the bunker shot and shanked the bunker shot, got up and down for bogey and then bogeyed the next two. I was kind of going, 'Oh, man, this is going the wrong way quick'.
"The fourth hole was kind of a pivotal hole for me. My caddie, he walks the entire golf course every week before I even play it and he gets all of his numbers. He got me a really good number to cover on that bunker on that wedge shot, hit a great wedge in there to eight feet and made birdie and that kind of turned things around and finished strong."