Kelly Grunewald, who aced the par-3 13th, and Daniel Summerhays both carded rounds of 3-under-par 68 Thursday. They share second place, one stroke ahead of Jason Allred, Chad Collins and Ben Bates.
Nallen, who started on the back nine, dropped in back-to-back birdies from the 12th to get on the board. He gave one of those strokes back as he bogeyed the par-three 17th on the Scarlet Course of Ohio State University Golf Club.
The 25-year-old recovered that lost stroke with a birdie on the first. Nallen moved to three-under as he converted a birdie try on the par-5 fourth.
Nallen tripped to another bogey on the par-5 sixth. He recovered that lost stroke with a birdie at the eighth and closed with a birdie on nine to end one clear of the field.
'The course is playing good, but a lot different than it did at the NCAAs,' said Nallen, who played on the track during the 2002 NCAA Championships. 'The greens are great. The bunkers are great. I was in a lot of them today. When we made the turn, the wind really started picking up, but I actually played better on the back nine.'
Nallen has struggled all year, making the cut in seven of 15 starts. He ranks just 87th on the money list and has posted only one top-10 finish.
'It's been an up and down year for me,' Nallen said. 'I've been working really hard lately on my short game. I've been losing shots because I have no confidence with the putter, and that is where it is at.'
Grunewald birdied three of four holes from the fourth, but in between he tripped to a bogey at the par-3 fifth. He reeled off four straight pars around the turn.
At the par-5 12th, he carded a birdie. Grunewald then aced the par-3 13th to jump to 5-under. However, he faltered to bogeys on 15 and 17 to close at 3-under.
'After you do something like that during the round, it's kind of hard because it's so exciting,' Grunewald said. 'It's such a high, but you need to maintain a steady pace mentally out here. After something like that it's hard to calm down, but I made a good par on the next hole, which was playing really hard.'
Summerhays posted three birdies and three bogeys on the back nine, his opening nine. Around the turn, he dropped another shot on the first. Summerhays flew up the leaderboard with four birdies in a five-hole stretch from the fifth.
John Riegger, who won the LaSalle Bank Open earlier this year, opened with a 1-under-par 70. He was joined by Brad Adamonis, Todd Fischer, Richard Johnson, Rick Price, Matthew Jones, Lee Williams and Deane Pappas in a share of seventh place.