John Jacobs, who shared the overnight lead, shot an even-par 70 and is alone in third place at 3-under-par 137. Mike San Filippo, who also held a share of the overnight lead, is at 2-under-par 138, alongside Tom Kite and Seiji Ebihara, who also fired a 4-under 66.
Morgan began his round on the 10th tee at the Aronimink Golf Club. He started in fine fashion as he dropped a 9-iron within inches of the cup at the par-4 11th and tapped in for the first of his six birdies.
Morgan ran off four consecutive pars, before attacking the par-5 16th. He knocked his 3-wood second shot just short of the green before rolling in a 20-footer for birdie.
He stumbled at the 18th, however. His second shot flew the green and after a nice chip, Morgan missed a short par-saving putt to make the turn at minus-1.
'I made one little boo-boo,' Morgan said. 'At my ninth, I missed a three-footer for par after making a good chip. Other than that I played pretty decently.'
Morgan drained a lengthy birdie putt at the par-4 second. However, he struggled at the next. His drive found heavy rough and his second shot hit the lip of a bunker in front of him. He dropped his third about 10 feet from the cup, but could not convert the par putt.
He then took advantage of two of the shortest holes on the course. Morgan birdied the 173-yard par-3 fifth from four feet, then sank another four-footer at the 397-yard par-4 sixth.
Morgan took care of the only other par-5, No. 9, on the course as well. His second came up just short, but he ran a 5-wood pitch shot within three feet of the hole and sank the birdie for a share of the lead.
'I played pretty well. I can't be too disappointed in my play,' said Morgan. 'I kept the ball in play pretty much and that's the key here. If you get it in the rough you are in serious trouble.'
Doyle had an up-and-down round with seven birdies, seven pars and four bogeys. He started his round with a bogey at the par-4 first after hitting his drive into thick rough but responded immediately as he sank a 15-foot birdie try at the next. However, Doyle struggled on the third green with a three-putt bogey.
Like Morgan, Doyle took advantage of Nos. 5 and 6. At the fifth, he nearly aced the par-three before tapping in for birdie. At the next, he dropped a 9-iron within four feet of the cup for his third birdie of the day.
Doyle again could not maintain his momentum as he dropped a shot at the seventh. He left his 8-iron short in a bunker and was unable to get up-and-down for par.
Doyle converted another birdie at the ninth. He knocked his second within 30 yards of the green and chipped within five feet of the cup. He sank that birdie try to head to the back side.
'I had four birdies on the front and turned at minus-1,' Doyle said. 'In a way, you're kicking yourself because you think you let the birdies slip away and didn't take full advantage of them.'
Around the turn, Doyle made one of the few birdies of the day at the par-4 13th. He missed the green at the par-3 14th and could not save par.
Doyle secured his share of the lead with back-to-back birdies at the 15th. He sank a 12-footer at the 465-yard par-4, and got up-and-down from just short of the green for birdie at the par-5 16th.
'I made some good ones on the back side,' said Doyle. 'It is probably not realistic to think you can make seven birdies a day over the next two days, but the key is to keep it in the fairway. I haven't seen anybody drive it in the tall stuff and make a par.'
Craig Stadler, who is making his Champions Tour debut, and defending champion Fuzzy Zoeller share seventh place at 1-under-par 139. Also tied with that pair are Wayne Levi and Jose Maria Canizares.
Bobby Wadkins, the final of three overnight leaders, struggled to a 2-over 72 and stands at even-par 140. He is joined there by Vicente Fernandez and Morris Hatalsky.
Jay Sigel, who is playing on his home course, came back with a 2-under 68 and is tied with five others at 1-over-par 141. Lanny Wadkins and Jim Thorpe survived right on the cut line of 7-over-par 147.
Among the notables missing the cut were Jack Nicklaus, Gary Player and Arnold Palmer, who all played here in the 1962 PGA Championship, the last major contested at this venue. Nicklaus ended at 10-over-par 150, with Player five shots further back at plus-15. Palmer ended at 19-over-par 159.
Jack Fleck, the oldest man in the field at 81, shot consecutive rounds of 16-over 86 to finish last among the field.