'I hit it good and putted it good and it comes out to 60,' said the 22-year- old, who turned pro after a successful college career at the University of Arizona. 'It's been a hectic couple of months. I'm trying to get everything settled. I feel like I've been on the go and haven't had a real rest.'
Brett Wetterich and Brent Schwarzrock shot matching rounds of 62 to tie for second at 10 under par. David McKenzie and Chris Anderson were one shot further back at 9-under-par 63.
While Wetterich and Schwarzrock seemed to be securing the overnight lead, along came Nallen.
Nallen played the back side first and kick-started his round with a birdie at the par-3 10th. He then eagled the par-5 12th and collected back-to-back birdies starting at the par-four 13th to quickly jump to 5 under.
He kept on rolling with birdies at 17 and 18 before an eagle at the par-5 second moved him closer to shooting a magical 59.
Nallen continued to score well with a birdie at the par-4 fourth and he made it two in a row with a birdie at the par-4 fifth. Nallen cooled off, however, with pars on each of his next three holes before a birdie at the par-4 ninth gave him a new tournament record.
Wetterich had the clubhouse lead alone before Nallen's blistering round. Wetterich made the turn at 4 under and collected a birdie at the par-3 10th to move to minus-5.
The 31-year-old, who won the Envirocare Utah Classic in August, tallied an eagle at the 12th and ran off three birdies over his last four holes to finish two behind Nallen.
Schwarzrock was also flirting with a 59 after five consecutive birdies from the first, his 10th, lifted him to 11 under.
'Actually, I was thinking about 58, just for a second,' said Schwarzrock.
Schwarzrock managed a bogey at the ninth, however, to join Wetterich in second.
Ricky Barnes and Steve Holmes shared sixth place at 8-under-par 64.