American Peter Uihlein extended his lead with a second round of 66 at the Shriners Hospitals for Children Open in Las Vegas.
Uihlein could not match his opening 63 but was two strokes clear of the field on 13-under after picking up birdies on five of the last seven holes on day two.
Starting on the back nine, compatriot Robert Streb moved up a place to tie for second with South Korean Whee Kim on 11-under after 36 holes.
Harold Varner III, Abraham Ancer and Bryson DeChambeau sit among a chasing pack on 10-under overall, while Jordan Spieth dropped into joint eighth on eight-under, making four birdies and a bogey in his round of 68.
Scotland's Martin Laird returned a second-round 68, rattling off four birdies in a six-hole stretch to get to seven-under.
Former US Open champion Graeme McDowell improved on his opening round of 70 to card 67, putting him in joint 31st on five-under overall.
Brian Gay was the biggest mover of the day.
The American started the second round three over par, but carded a blemish-free second-round 65 to move above the cut line on three-under before play was suspended due to darkness.