Summerhays won the Children's Hospital Invitational two months ago, then turned pro and joined the tour eight days later. On Friday, he shot a 6-under 66 to become the only player among eight first round co-leaders to remain on top at Shadow Hills Country Club.
Richard Johnson fired an 8-under 64 and joined Summerhays in the lead at 12-under 132.
Jason Day had a 67 and was alone in third place at 11-under 133, while Jon Turcott (66) was a shot further back at 10-under 134. Mike Heinen (69) and Kelly Grunewald (69) were tied for fifth place at 9-under 135.
B.J. Staten carded a 10-under 62 to break the course record by a shot and climb 110 places into a tie for seventh. He was joined at 8-under 136 by three other players.
Summerhays has now made five cuts in six starts since turning pro on July 23, eight days after the win in Columbus changed his life.
'I've learned so much out here since turning pro,' said Summerhays, who had six birdies on Friday for his second straight bogey-free 66. 'I'm getting to know a lot of the guys. It's such a test out here. You get to see so many great golf courses. You learn new shots. The fields are deep and week-in week- out it's a great challenge to come out here and play.'
Johnson birdied his last five holes to shoot 64 and tie Summerhays heading into the third round. He is No. 11 on the money list this season despite still chasing his first career win.
'It's nice to play well this year. Winning will happen if you keep playing well,' said Johnson, who has two runner-up finishes in his last four starts.
He made eight birdies in his bogey-free round.
'I've got a pretty good consistent thing I'm doing with my golf swing,' said Johnson 'I'm working on the same things. I'm not fiddling around and I'm putting pretty well.'
The cut line fell at 2-under 142 with 77 players moving on to the weekend.