The 29-year-old Summerhays, who finished ninth in last week's Greenbrier Classic, birdied four of the first five holes and finished with 10 overall for the day.
He ended the third round at 19-under 194, two better than David Hearn.
Hearn carded a 64, moving into second place at minus-17.
Defending champion and overnight co-leader Zach Johnson fell into sole possession of third place at 16-under 197 with a round of 67.
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