Tom Kite and Sammy Rachels share third place at 7-under-par 65. Wayne Levi, the defending champion, is one stroke behind that duo at minus-6.
Stadler, who has picked up five wins on the Champions Tour in a little over one year, did not get off to a good start. He faltered to a bogey at the par-3 fourth. However, he came right back with a 12-foot birdie putt at the next.
The 51-year-old knocked a 5-wood to 5 feet to set up eagle at the par-5 sixth. He tripped to another bogey at the following hole, this one due to a three-putt.
Stadler drained a 12-foot birdie try at the ninth and nearly holed his pitching-wedge at the 10th, before kicking in his birdie putt to move to 3 under.
He then ran home a 5-foot birdie putt at the par-5 12th and closed with a flourish. Stadler sank a 2-foot birdie try at the 15th and then drained a 61-footer for birdie at the next. He made it three straight with a birdie at the 17th and dropped in a 5-footer at the last to close with four consecutive birdies.
'I drove it in every fairway,' Stadler said. 'It was very enjoyable to watch putts go in. This is a good course for me. Not many holes that go right-to-left. I can curve it off these bunkers. If you hit the ball well on this course, you can score.'
Purtzer, who won the Toshiba Senior Classic earlier this year, opened with a short birdie putt at the first. He then two-putted for birdie at the par-5 third. Purtzer dropped a pitching-wedge 1 foot from the cup to set up birdie at the seventh.
Around the turn, Purtzer turned it on and stuck a sand-wedge within a foot of the hole for birdie at the 10th. After a 5-foot birdie putt on 11, he pitched his third to 2 feet for birdie at the 12th.
Purtzer rolled in a 15-foot birdie putt at No. 13 and capped a run of five straight birdies with a 3-foot birdie putt at the 14th. He parred his final four holes to share the overnight lead.
'I like the course. I had some success here last year,' said Purtzer, who tied for 10th in 2003. 'I got off to a good start. When you birdie the first hole, you know it might be a good day. The greens are as good as I've putted on.'
Hale Irwin, a three-time winner at this event, fired a 5-under-par 67 to grab a share of sixth place. He is joined there by Eamonn Darcy, Jim Albus, Jim Thorpe, Fuzzy Zoeller and Mark McNulty.
Vicente Fernandez, who won this event in 1996, and Morris Hatalsky, who shared second place last year, are one stroke further back at minus-4. They are tied there by Doug Tewell, John Jacobs, Don Pooley and Des Smyth.