Xander Schauffele carded a superb eight-under 64 to grab a three-stroke lead at the halfway mark of The CJ Cup at Shadow Creek.
Schauffele, a four-time winner on the PGA Tour, moved into 14 under on the back of a bogey-free second round in Nevada on Friday.
The American started his round on the back nine, and made seven birdies in eight holes – including six straight from the 13th.
His back-nine 29 marked his career-low nine-hole score on the PGA Tour.
Schauffele is three shots clear of Tyrrell Hatton, who held a one-stroke overnight lead.
Hatton, who won the BMW PGA Championship on the European Tour last week, endured a mixed round with seven birdies and three bogeys.
Russell Henley is outright third at 10 under after also shooting a 68.
Brooks Koepka and Rory McIlroy improved in the second round to sit at two under in a tie for 26th.
Koepka managed a four-under 68 after his opening 74, while McIlroy fired a 69.
Justin Thomas, the defending champion and two-time winner of the PGA Tour event, is in contention after carding a 66 to get to six under.
Talor Gooch sits outright fourth at nine under after an impressive 65, and he is a stroke ahead of Jason Kokrak (66) and US PGA Championship winner Collin Morikawa (65).
Meanwhile, Jordan Spieth's struggles continued as he shot a second straight 74 to be at four over and in a tie for 61st.
Winless since 2017, Spieth has missed three consecutive cuts and has just two top-10 finishes in 16 events this year.