Former U.S. Open champion Corey Pavin, Darron Stiles, Memorial winner Carl Pettersson, Camilo Villegas, Todd Fischer and Ron Whittaker are knotted in fifth place at minus-4.
On the back nine, Verplank recorded his first birdie at the par-4 12th. He drained an 11-foot birdie putt on the last to become the lone co-leader from the afternoon tee times.
'I only missed one fairway and I hit a lot of greens,' said Verplank. 'I like this golf course okay. I haven't played here that often, but it seems to fit my game.'
Verplank is finally returning to form as he has been hampered by a shoulder injury. A couple of high finishes could vault him up from 16th on the U.S. Ryder Cup points list.
'If I play like I'm used to playing here in the next couple of months, I'll have a legitimate shot to make it,' admitted Verplank. 'I'm just turning the corner now.'
Slocum, who has won in each of the last two seasons, started on the back nine on Thursday and parred his first three holes. At the par-5 13th, Slocum tried to get home in two, but blocked his second right. He knocked his third to 35 feet, but rolled in the birdie try.
At the reachable, par-4 15th, Slocum missed the green left, then chipped to 6 feet, where he canned the putt. He closed his opening-nine holes with a 25-foot chip-in birdie from the fringe at 17.
On his second nine, Slocum ran home a 5-foot birdie putt at the first. He hit a 5-wood to 8 feet at the par-3 fifth to set up birdie then parred out for his share of the lead.
'Today I started off with quality good shots, then made some putts and got a little bit of confidence,' said Slocum, who owns only one top-10 this season. 'Overall it was just a good day.'
Lonard three-putted the 13th green for a bogey, but caught fire after that. At the 14th, Lonard hit a 6-iron to 3 feet to set up birdie, then kicked in another birdie putt from the same length at 15. The Australian converted a 30- footer for birdie from the fringe at 16, then polished off his fourth birdie in a row from 20 feet at the 17th.
At the second, Lonard hit a wedge to 4 feet and holed the birdie putt. He reached 5 under par with a 20-footer with a birdie at the par-3 eighth hole.
'It was one of those rounds where I hit a lot of greens, a lot of fairways and holed a couple of long putts and maybe missed one I probably should have got,' acknowledged Lonard. 'All in all, it was pretty safe.'
Frazar started on the back nine Thursday and birdied two in a row from the 13th. He bogeyed No. 15, but reclaimed the lost stroke with a birdie at the 17th.
Frazar tallied three birdies in a four-hole span in the middle of his second nine to reach minus-5.
Justin Leonard, Kenny Perry and Zach Johnson headline a group of players tied for 11th place at 3-under-par 67.
Defending champion Brad Faxon struggled mightily on Thursday. He shot a 4-over-par 74 and is part of group tied for 137th place.