One of three leaders overnight, Summerhays fired a flawless four-under-par 67 in blustery conditions on the El Camaleon course to post a 12-under total of 201.
Compatriot Chris Stroud was alone in second after carding a 68, with fellow Americans Michael Allen (66) and Will Claxton (71) a further two shots back at eight under.
"Like yesterday, I drove the ball very well," Summerhays told reporters after carding four birdies to take control of the tournament. "I was feeling really comfortable with it, swinging aggressively, and I was never too far out of position."
Summerhays, who won the 2007 Children's Hospital Invitational on the satellite Nationwide Tour as an amateur, has yet to win a tournament since turning professional.
The 28-year-old from Utah holds the 54-hole lead for the first time on the PGA Tour and said he would try to replicate successful playing strategy this week in Sunday's final round.
"Nothing's going to change," Summerhays added." I'm going to swing hard at that driver ... and I'm going to continue to hit the knock-down iron shots that I've been hitting all week.
"I'm really looking forward to the opportunity tomorrow to take the lead into the final round of a PGA Tour event and see how I perform."
(Writing by Mark Lamport-Stokes in Marana, Arizona; Editing by Greg Stutchbury)
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