Nick Cassini, Steven Alker and last week's rain-shortened Oregon Classic winner Jeff Gove are knotted in fourth at minus-7 at the Cattail Course at Whirlwind Golf Club.
Gronberg, who plays primarily on the PGA Tour, opened on the back nine and drained a 45-foot birdie putt at the 10th. He added birdies at 12, 15 and 16, but cruised up the leaderboard with a 60-foot eagle putt on the 17th to make the turn at 6-under 30.
'Making a 45-footer for birdie on the first hole was a nice way to get the round started,' said Gronberg, who won the 2003 PGA Tour Qualifying Tournament.
The Swede added three birdies and a bogey on his second nine to grab his share of the lead.
'I played really well today from tee to green and then it was an exceptional day on the greens,' said Gronberg. 'I had a couple of misses, but overall it was a very good putting day for me.'
Schwarzrock did not make a par until the fifth hole, as he started his round with three birdies and a bogey. He collected three more birdies on his front side.
He birdied No. 10 and eagled the 12th to stake his claim to the lead, a position he has been in this season. Schwarzrock was in first after this year's Knoxville Open, but tied for 45th.
McKenzie also played the course in the opposite order, but tallied five birdies through his first 13 holes. On the par-4 fifth, McKenzie drove the green and rolled in the 40-footer for eagle.
He birdied his eighth hole to join Gronberg and Schwarzrock in a tie for the lead.
'I played well today,' said McKenzie, who is 25th on the Nationwide Tour money list. 'It is probably one of the first times this year I feel like I've struck it well. I've got four weeks to make whatever I have to make.'
Bubba Watson, Doug LaBelle II, Troy Matteson, Dicky Pride, Jeremy Anderson, Johnson Wagner and Jerry Smith are knotted in seventh place after opening with 6-under-par 66s.