Walter Hall and 2003 Senior PGA Champion John Jacobs share second place at 9-under-par 135. John Harris, Tom Kite and Jerry Pate are one stroke further back at minus-8.
Purtzer opened with a 6-foot birdie at the par-4 first. After four straight pars, the 52-year-old stumbled to a bogey at the par-4 sixth. He erased that mistake with a 12-foot birdie putt at No. 8.
Around the turn, Purtzer birdied the par-5 10th to get to 9 under. He came right back to sink a 12-foot birdie the 11th. Purtzer, who won the Toshiba Senior Classic earlier this year, parred the next six holes at Nashawtuc Country Club.
Purtzer then drained a birdie at the par-5 closing hole to move two strokes clear of the field entering the final round.
'It'll be an interesting day tomorrow,' Purtzer said. 'My swing was a little loose today. I didn't feel quite as good today. I'll need to make some putts and get off to a good start on Sunday.'
Jacobs tripped out of the gate with a bogey at the par-3 second. However, he came right back to birdie the third. The five-time winner on the Champions Tour made it two straight as he dropped in a birdie at the fourth.
He was not done. Jacobs drained his third birdie in a row at the fifth to climb to minus-6. He then parred the next four holes before dropping in back-to-back birdies from the 10th. He moved to 9 under and a share of second place with a birdie at the 14th. He parred out to remain there.
'When you've won events before, you can win anywhere,' said Jacobs. 'I like Boston. I don't know why I haven't done well here. I was a little discouraged with my finish, but I'm still in the action.'
Hall posted three birdies in a four-hole stretch from the fourth to get to minus-8. Later, he converted birdie tries at the 12th and 14th to move to 10 under and a share of the lead.
However, Hall faltered to a bogey at the 16th to drop back to 9 under. He parred his final two holes to join Jacobs in a tie for second.
Jay Sigel, Craig Stadler and Doug Tewell share seventh place at 7-under-par 137.
Allen Doyle, the 2003 champion, is one stroke further back at minus-6. He is joined there by Graham Marsh, Mike McCullough and Dana Quigley. Isao Aoki, John Bland, Eamonn Darcy, Ed Fiori, Tom Jenkins, Sammy Rachels and Des Smyth follow that group at 5-under-par 139.