Bruce Lietzke is one stroke behind Purtzer at 8-under-par 64. Don Reese stands alone in third place at minus-6. Defending champion Tom Kite is one of nine players at 5-under-par 67.
Purtzer opened with a birdie on No. 11, his second. He made it two straight with a birdie on the 12th. After five straight pars, the 2004 Toshiba Senior Classic winner posted a birdie on the 18th at the TPC of the Twin Cities.
Around the turn, Purtzer birdied the second to climb to minus-4 and he made it two in a row with a birdie on the third. After a pair of pars, Purtzer moved to minus-6 with a birdie on the par-5 sixth.
The 53-year-old came right back with a birdie at seven. Purtzer then holed his 4-iron tee shot at the par-3 eighth for a hole-in-one to close a bogey-free round at minus-9. It was sixth ace in tournament history and helped Purtzer tie the course record of 63.
'Someone dropped a bottle as I was ready to hit on No. 8 so I backed off the ball,' said Purtzer. 'I stepped back in, hit it, and it came off the club just the way I wanted it to. I'm as surprised as anybody it went in. You don't ever expect a tee shot to disappear into the hole.'
Purtzer has some mixed memories at this event. He led entering the final round each of the last two years, but failed to win either time.
'I like this golf course, but I've managed to fritter away a win here each of the last two years,' said Purtzer, who tied for third last year and shared 10th in 2003. 'I figure one of these years I may sneak through and win here.'
Lietzke also opened on the back nine and started with a birdie on the 10th. He posted his second birdie on the par-4 14th. The 2003 U.S. Senior Open champion tripped to a bogey on the 17th.
He got that stroke back with a birdie at the next. Lietzke, who won this crown in 2001, made it two in a row around the turn with a birdie on the first.
The 54-year-old jumped to minus-5 when he holed out for eagle on the par-5 third. Lietzke kept rolling with a birdie on four. He then birdied the sixth and closed with a birdie on the par-4 ninth to move into second place.
'I can't think of a better course for me and my game,' said Lietzke. 'This is where I had my first Champions Tour win, so there are great memories here. This is a course I always look forward to playing.'
Kite was joined in a tie for fourth place at minus-5 by David Eger, John Ross, Brad Bryant, John Harris, Graham Marsh, Mark McNulty, Gil Morgan and Lonnie Nielsen.
Leonard Thompson, who won this title in 1998 when the tournament was shortened by rain, opened with a 4-under-par 68. He was joined there by 2003 co-runner-up Morris Hatalsky, Jose Maria Canizares, Bobby Wadkins, Bob Gilder, Joe Inman and last year's runner-up, Craig Stadler.