Sheftic, 34, of Ambler, Pa., holds a one-stroke lead over Kyle Flinton of Oklahoma City heading into Tuesdays third round, which will be played at the par-71, 7,493-yard Twin Warriors Golf Club.
Sheftic was at 7-under 135 for two rounds.
The 312 original competitors took turns playing a round at Twin Warriors and one at the par-71, 3,551-yard Santa Ana Golf Club, which has been the easier of the two courses this week.
The top-87 scorers will play the final two rounds at Twin Warriors. The top-20 finishers are eligible for the PGA Championship at Hazeltine National Golf Club in August.