Dan Olsen carded a 3-under 69 at Panther Lake to join Michael Allen, John Riegger and Danny Ellis in a tie for second at 11-under-par 277. Daniel Chopra, who shared the lead with Gronberg after the third round, was one shot further back at 10-under-par 278.
With four rounds completed of this marathon event that will send the top-30 and ties to the PGA Tour in 2004, Gronberg, who has held at least a share of the lead each day, has begun to pull himself away from the field thanks to steady play.
'A lot of things can happen, but hopefully with maturity I'm going to try and focus on keeping the same routine tonight and in the morning, try to hit the first fairway tomorrow, and take it from there,' said Gronberg. 'I'm not going to allow myself to get too far ahead. There are two rounds remaining but I'm very confident.'
The Swede coasted at even-par through the front nine and picked up his lone birdie at the par-4 12th.
Gronberg continued his unblemished round and parred his way in for his second straight bogey-free round.
'I'm very pleased,' said Gronberg. 'I thought I was making a bogey on the last hole. I duffed my first putt way short. I was looking to make birdie from where I was. I left it six feet short, and I was fortunate to make the putt. Obviously on a day like today where you make no bogeys you're going to advance.'
Olsen, who at one time worked as a Christmas tree salesman, played the back side first and collected a bogey at the par-416th.
He recovered on the inward half, however, with back-to-back birdies from the par-4 first before running off two consecutive birdies starting at the par-5 seventh.
Todd Hamilton, Scott Hend and David Branshaw share seventh place at 9-under-par 279. Kris Cox and Olin Browne followed at 8-under-par 280.
Arjun Atwal shot a 2-under 70 to join Dennis Paulson, Trevor Dodds, Russ Cochran, Wes Short, Danny Briggs and Kevin Na at 7-under-par 281.