Tom Jenkins carded a 66 to climb into second place at 9-under-par 135.
Doug Tewell, the 2001 champion here, is one stroke further back at minus-7. He was joined there by Brad Bryant, Mark James and Weibring.
Morgan, a two-time winner of the Tradition, made a nice par save at the second as his 8-foot putt dropped into the cup. He came right back with a 10-foot birdie putt on the third to get to minus-4.
The 58-year-old knocked his third shot to the par-5 sixth to 20 feet and ran home that putt for birdie. Morgan parred his next four holes around the turn. He drained a 35-foot birdie putt on the 11th to get to minus-6 on the South Course at The Reserve Vineyards & Golf Club.
Morgan stuck a sand wedge within 9 feet at the 12th and drained that for his second straight birdie. The Oklahoma native birdied 13 from just inside 9 feet to cap a run of three straight birdies.
The 23-time winner on the Champions Tour got up and down for birdie on the par-5 15th.
Morgan, who holds a Doctor of Optometry degree but has never practiced, found the putting surface at the par-5 16th in two. He sank his eagle putt to jump two strokes clear of the field.
'It turned out to be an exciting day,' said Morgan. 'I didn't get off to a great start. Those three straight birdies really got me going. This is the last major and you want to do well.'
Jenkins got going with birdies on the third and fifth to get to minus-5. He stumbled to a bogey at the par-3 seventh, but rebounded with a birdie on eight.
The 57-year-old got to minus-6 with a birdie on the 10th. After a pair of pars, Jenkins holed out for eagle on the par-4 13th.
'That landed in the bottom of the cup and stayed there,' said Jenkins. 'That was a big bonus on that one. I was waiting, when I heard the sound, to see where the ball was going to fly out to.'
Jenkins, who owns six Champions Tour wins, faltered to a bogey on 14. He came right back to birdie the 15th and 16th, both par-5s, to get within two of Morgan.
Harris managed a 1-under 71. He stands alone in ninth place at 6-under-par 138. Quigley posted an even-par 72 is shares 10th place at minus-5 with Jerry Pate (68) and Dave Stockton (69).
Defending champion Craig Stadler and Bob Gilder each posted rounds of 4-under 68 to share 13th place at 4-under-par 140. They were joined there by Dave Barr, Bob Gilder and Mark Johnson.