Rickie Fowler followed up an opening 64 with a 65 on Friday as he prised a one-shot lead at the Waste Management Phoenix Open.
The American was level with compatriots Justin Thomas and Harold Varner, but strung together four birdies over the last four holes to finish six under for the day and 13 under for the tournament.
It could have been a bigger lead, but Fowler dropped a shot at the par-4 ninth.
Thomas sits second after a round of 66, mixing two birdies with a bogey on the 13th, 14th and 15th in Scottsdale.
Branden Grace and Trey Mullinax sit a shot further back and tied for 3rd, while Scotland's Martin Laird is in a two-way tie for seventh after carding a 68.
Varner had a torrid second round, carding 71 and dropping to tied 11th, while Japan's Hideki Matsuyama, bidding to win the tournament for a third time, is tied 24th on five-under.
Phil Mickelson's record 30th appearance in the event was brief, with a second round of 75 meaning he missed the cut.
