Lonnie Nielsen was the only other player to reach double figures under par before bogeying the last to close a round of 4-under 68. He stands at 9-under-par 135 and shares second place with Craig Stadler (67), David Eger (68), Bruce Lietzke (71) and Hajime Meshiai (66).
Defending champion Tom Kite is one stroke further back at minus-8.
Purtzer, who tied Don Pooley's course record of 63 on Friday, opened with a birdie on the par-5 third. He moved to minus-11 two holes later as he birdied the fifth at the TPC of the Twin Cities.
The 53-year-old got to 12-under when his birdie putt on the seventh found the bottom of the cup. Purtzer, a two-time winner on the Champions Tour, parred his next four holes around the turn.
Purtzer, who has led entering the final round each of the last two years, but failed to win either time, birdied the par-5 12th to get to 13-under. After four more pars, he faltered to a three-putt bogey at the 17th to end at minus-12.
'That was a disappointing finish,' said Purtzer. 'I felt like I played pretty solid. What happened a year ago doesn't have anything to do with what's going to happen tomorrow.'
Nielsen ran off three consecutive birdies from the third to jump to minus-8. He parred his next seven holes before birdieing the par-3 13th. The 52-year-old got to 10 under with a birdie on 16. However, he tripped to a bogey at the par-5 closing hole to share second place.
Meshiai birdied the second and third, but bogeyed the fourth. He dropped in consecutive birdies from the sixth to move to minus-6. The 51-year-old birdied 12, 16 and 18 to finish at 9 under.
Stadler, who finished second here last year, played the front nine in one- under with a pair of birdies and a bogey. He birdied the 11th to move to 6 under. The 2004 JELD-WEN Tradition champion closed with three birdies over his final four holes.
Eger got off to a tough start with a bogey on the first. He got that shot back with a birdie on No. 3, but he tripped to another bogey on the fourth.
The 2003 MasterCard Classic winner birdied six, seven and nine to turn at 7 under. Eger picked up birdies on 13 and 18 to end at minus-nine.
Lietzke, who entered the round one shot behind Purtzer, stumbled out of the gate with bogeys on each of the first two holes. The 2003 U.S. Senior Open champion fought right back with birdies on three and four. He parred his next seven holes before sinking a birdie try on the 12th. Lietzke, the 2001 winner here, parred out to remain at minus-9.
Kite, who shot a 3-under 69, is joined in a tie for seventh place by Jim Ahern, Morris Hatalsky, D.A. Weibring and Gil Morgan. Andy Bean and Brad Bryant are one stroke further back at 7-under-par 137.