Play is split over the Panther Lake and Crooked Cat courses at Orange County National. The tournament is 108 holes, with a cut after the first 72, and the top 30 and ties after Monday's final round, earn their PGA Tour cards for the 2004 campaign.
Jeff Freeman (68), Bradley Hughes (69), first-round co-leader Danny Briggs (70), Jon Mills (68) and Kevin Na (67), the youngest player in the field, are tied for second place at 7-under-par 137.
Daniel Chopra, the third co-leader from Round 1, shot a 1-under 71 on Thursday to sit alone in seventh place at minus-6.
Gronberg started at No. 1 at the Panther Course Thursday and mixed four birdies and three bogeys for a 1-under-par 35. He was still 1 under on his round after a birdie at 10 and a bogey at 15.
But the Swede rebounded late in his round by birdieing two of his final three holes to cruise past the logjam into the lead.
'It was very interesting,' Gronberg said of his round. 'A couple of birdies, some bogeys and I was struggling. I was very fortunate to have a good round.'
Gronberg has made a living on the European Tour since 1994 and has done so with some success. He has tallied four victories, including this year's Italian Open.
'I have a very comfortable life over in Europe,' said Gronberg. 'Sometimes, maybe that holds me back. Maybe your not pushing yourself. If I get my tour card over here, I probably need to play all my tournaments over here.'
Wes Short (69), David Branshaw (70), Todd Hamilton (70), Olin Browne (70), Frenchman Thomas Levet (67), Kenichi Kuboya (69) and Scott Hend (69) are tied for eighth at 5-under-par 139.
Among the notable players trying to earn their cards are: Per-Ulrik Johansson, who shot a 2-over 74 on Thursday, is tied for 71st at even-par 144 and two-time Ryder Cupper Steve Pate, who also carded a 74 in the second round and is a stroke further back at plus-1.