Tom Kite used a 3-under 69 to move into second place at 10-under-par 134. Craig Stadler, who shared the overnight lead with Purtzer, posted a 1-under 71. He stands alone in third place at minus-9 with Sammy Rachels one stroke further back.
Wayne Levi, the defending champion, managed an even-par 72. He stands at 6-under-par 138 alongside Vicente Fernandez and Des Smyth, who both carded rounds of 2-under 70.
Purtzer took command early in the round, but struggled midway through before a late rush gave him the overnight lead at TPC of the Twin Cities.
The 52-year-old Purtzer dropped in back-to-back birdies from the second to get to minus-10. He came right back with an 8-foot birdie putt at the par-4 fifth. He ran off four straight pars to move to the back nine at 11 under.
Purtzer fell back to minus-10 with a bogey at the par-4 10th. Kite playing ahead of Purtzer dropped in a birdie at the 12th to join Purtzer in the lead at 10 under.
However, things quickly turned back in Purtzer's favor. He two-putted for birdie at the 12th to move to 11 under, while Kite tripped to a bogey at the 13th.
That duo again ended in a tie for the lead at 10 under as Purtzer bogeyed the 13th and Kite birdied the 15th from 2 feet. Purtzer settled in with three straight pars from the 14th.
The 2004 Toshiba Senior Classic turned it on down the stretch. He rolled in a 3-foot birdie at the par-3 17th and came right back to two-putt for birdie at the 18th for a two-shot cushion.
'I knew when we started scores weren't going to be as low,' Purtzer said. 'I got off to a great start. That was a key save for bogey at 10. I would have been upset if I would have double-bogeyed that 10th.'
Kite had spread two birdies over his opening 11 holes before grabbing a share of the lead at the 12th. After again grabbing a share of first at the 15th, Kite slipped to a bogey at the par-3 17th. He managed to close with birdie to get within two shots of Purtzer's lead.
'I'm pleased with the position I am in,' Kite said. 'I hit three very poor iron shots on the back nine. I played a solid round and drove the ball well though. It was a difficult day.'
Dana Quigley and Jay Sigel posted matching rounds of 2-under 70 to move to 5-under-par 139. They are joined there by Morris Hatalsky and Doug Tewell, who each shot 71s, and Jim Thorpe, who carded an even-par 72.