The start of the round at TPC San Francisco Bay at Stonebrae was delayed more than two hours due to fog. The round will resume Friday at 11:00 a.m. (ET) with start of the second round pushed back until 12:30 p.m.
Chad Ginn is alone in second place at minus-4, but he has only completed 11 holes of his opening round.
Craig Barlow, Dennis Paulson and Garth Mulroy finished the first round at 3-under-par 68. They were joined in a share of third place by Nick Flanagan, who is minus-3 through nine holes.
Demsey, who is looking for his first Nationwide Tour title, started on the back nine and collected his first birdie on the par-4 11th. He made the turn at minus-3 thanks to birdies on 16 and 18.
Around the turn, Demsey carded his fourth birdie on the par-5 third. He dropped in back-to-back birdie efforts from the sixth to get to 6 under before parring his final two holes to close out a bogey-free round.
'As cold and windy as it was today, before I teed off, I would have guessed even-par would have been a pretty good score,' Demsey admitted. 'I tend to do pretty well in the wind. I feel like I can control my trajectory pretty well. Everything was working. It didn't feel perfect, but everything was close.'
Ginn birdied the third and fifth to get his round going. After stumbling to a bogey on the sixth, he bounced back with a birdie on the par-5 seventh.
The 38-year-old connected on back-to-back birdie efforts from the 10th to move within two of the lead before play was called for the night due to darkness.
Gavin Coles, who won the Louisiana Open last year, posted a 2-under 70. He shares sixth place with Jim McGovern, John Kimbell, Clay Ogden, Brian Smock and Dustin Risdon. Smock has played 13 holes, while Risdon completed 12 holes of the first round.