Summerhays, a 23-year-old senior at Brigham Young University, shot a 1-over 72 in the third round Saturday but maintained the share of the lead he held overnight.
Chris Nallen had a 1-under 70 to join Summerhays on top at Ohio State's Scarlet Course.
They stood at 4-under 209 for a two-shot lead over two players.
Summerhays, one of two overnight co-leaders, held a two-shot lead at one point Saturday after making birdies at two of his first four holes. Later, he bogeyed the eighth and double-bogeyed the 10th to fall a shot off the pace.
Two birdies mixed with two bogeys gave Summerhays an even-par back nine -- good enough to vault him back into a share of the lead.
'When you give [shots] back on that course as tough as it is, it is tough to get them back,' said Summerhays. 'But I said, 'you know what, I've hit almost every single shot solid, so why not keep making good, aggressive swings?'
'It's hard to get frustrated and lose control, so I just regrouped and held it together.'
Nallen, who won the 2004 Gila River Classic, birdied his first two holes out of the gate Saturday, then made 11 consecutive pars before posting another birdie at the 14th,
He double-bogeyed the 15th and bogeyed the 16th, but came back with a birdie at the 18th -- an up-and-down round, for sure.
'But I didn't really make any bad decisions,' Nallen reasoned. 'I just didn't execute shots the way I wanted to. You've got to think your way around this golf course. You've got to hit and produce shots, and it just brings out every part of your game.'
Chad Collins (70) and Richard Johnson (72) shared third place at 2-under 211, and there were 14 players overall within five shots of the co-leaders.