Leonard Thompson, the first-round leader, fired a 1-under 71 and is tied for fourth place with Jerry Pate and John Bland, both of whom shot 69 in round two. The trio is knotted at 7-under-par 137.
Temperatures were in the mid-90s on Saturday, so tournament officials allowed the players to ride in carts.
That came in handy for Purtzer, who said that the heat helps his body loosen up. Purtzer injured his back in a 1992 dirt bike accident, but nothing stopped the two-time winner on the Champions Tour.
He lipped out a 15-footer for birdie at the first hole, then sank a 5-footer for birdie at the third. The shot of the round came at the seventh for Purtzer. He holed out a 9-iron from 147 yards out for an eagle-2.
Purtzer kept the momentum going from the hole-out eagle. He knocked a 5-iron to 10 feet to set up birdie at the eighth, then played a wedge inside three feet for another birdie at the par-5 ninth.
'That eagle at No. 7 really jump-started things,' said Purtzer. 'I kept things going on the next couple of holes with birdies.'
Purtzer parred his first four holes on the back nine, but returned to his birdieing ways at the 14th. He knocked a pitching wedge to 8 feet, then made it two in a row at 15 when his 15-footer for birdie fell at the short, par-4.
The 53-year-old reached the green in two at the par-5 closing hole. He two-putted from just inside 20 feet for his sixth birdie of the round and is the only player in the field without a bogey.
Purtzer trailed Thompson by five and had a strategy heading into Saturday.
'I was trying to cut the lead in half or better,' said Purtzer, who held the 36-hole lead of this event last before Craig Stadler overcame him. 'I accomplished that and I'm pleased where I am.'
Eaks was 3-under through 11 holes, but caught fire down the stretch. He drained a 10-footer for birdie at 13, then kicked in a short birdie putt at 15. Eaks collected his second birdie in a row from a little over 9 feet at No. 16.
Eaks knocked a 5-wood over the green at 18 and his chip stopped 18 feet from the hole. He ran home that birdie putt to get within one of Purtzer's lead.
'Tom is really playing well now,' said Eaks, who played with Purtzer on Saturday. 'It was fun to watch him today. Teeing off early, I think we got a little bit of a break today.'
McNulty collected three birdies through his first 14 holes, then fell down the leaderboard. He three-putted from 30 feet for a bogey at 15, then made a mess of No. 16 for back-to-back bogeys.
McNulty hit a 7-iron inside 4 feet to set up birdie at the par-3 17th. He two-putted from 11 feet for another birdie at the last to get into second.
'My caddy told me to just re-focus after 16 and made three good shots after that,' said McNulty. 'He said to take a few deep breaths and refocus.'
Andy Bean, who rounded out the threesome with Purtzer and Eaks, shot a 5- under 67. He is tied for seventh with Bruce Lietzke (66), D.A. Weibring (69), Hale Irwin (68) and John Harris (71). The group is tied at 6-under-par 138.
Stadler carded a 4-under 68 and is tied for 18th at minus-4.