American Andrew Landry edged one shot ahead of Jon Rahm to take the lead at the halfway stage of the CareerBuilder Challenge in California.
Landry, 30, started the day one behind Rahm after a first-round 63 but nosed in front with a seven-under round of 65 at the PGA West's Jack Nicklaus Tournament Course, moving to 16 under 128 after 36 holes.
The Texan only managed two birdies on the front nine but a run of five birdies in six holes from the 11th saw him surge up the leaderboard as he made the most of the calm conditions.
World No 3 Rahm followed his opening round of 62 at La Quinta with a 67 on the Nicklaus layout.
The Spaniard recorded four birdies and two bogeys on his first six holes after starting on the 10th, and then followed a run of six successive pars with birdies at the fourth, seventh and eighth.
"I had close birdie putts over the last four holes and made two of them, so I think that kind of clicked," said Rahm, who has played the par fives in nine under so far this week.
"It's tough to come back because I feel like I expected myself to go to the range and keep just flushing everything like I did yesterday. Everything was just a little bit off."
Jason Kokrak, joint-second after day one, now sits in third on 14 under following a 67 at Nicklaus, while his fellow Americans Zach Johnson, Michael Kim and Martin Piller are a shot behind on 13 under.
Johnson and Kim fired 64s at Nicklaus, and Piller had a 67 on the Pete Dye-designed Stadium Course.
Defending champion Hudson Swafford is tied for 19th after carding a four-under 68 at Nicklaus, after a 66 to open the tournament.
Scotland's Russell Knox moved 54 places up the leaderboard into a tie for 10th with an eight-under round of 64 at Nicklaus, with five of his eight birdies posted on the front nine.
Phil Mickelson, playing in his first tournament since late October, followed his opening 70 at La Quinta with a 68 at Nicklaus to get to six under.
After perfect conditions over the first two days, gusts of up to 30mph are forecast for Saturday when Landry and Rahm will both be in action on the once-feared Stadium Course.
"It's going to be a tough day," Landry said. "I know that golf course can get a little mean. Especially, those last three or four holes coming down the stretch.
"But we're in a good spot right now. I played two good rounds of golf, bogey-free both times, and it's just nice to be able to hit a lot of good quality shots and get rewarded when you're making good putts."