The 39-year-old Gronberg, a four-time winner on the European Tour, shot a 5-under 67 in the third round in the morning, then added a 7-under 65 for a tournament-record 19-under 269 total in the Nationwide Tour event at The Country Club at Woodmore.
I started today five shots behind the leader, Gronberg said. You need 36 holes to play well, to kind of squeeze by, and thats what happened.
I got very hot at times today, especially in the final round. There were a couple of stretches where I played very, very good golf.
Gronberg was set to play in a U.S. Open sectional qualifier Monday in New York before heading to Tennessee for the PGA Tours St. Jude Classic.
When youre playing well, you want too keep playing, said Gronberg, who tied for 11th last month in the PGA Tours Texas Open. Obviously, its easy to say now, but I dont feel that tired. Hopefully, Ill feel just as good and as happy as I was today.
Making only his second Nationwide Tour start of the year, Gronberg earned $121,500.
Damron finished with a 67, and Bolli had a 70.
Mexicos Esteban Toledo (66) was fourth at 12 under, and Tom Scherrer (72) and Swedens Oskar Bergman (73) followed at 11 under.
Michael Sim, a two-time winner this year, shot a 73 to tie for 10th at 9 under. The Australian is a victory away from an in-season promotion to the PGA Tour. He earned $17,550 to increase his tour-leading total to $388,067.