Allen Doyle, who has top-10 finishes in each of the season's first two majors, carded a 6-under-par 66 and shares second place with Fred Gibson, Tom Kite, Mike McCullough and Doug Tewell.
Morgan got going with a birdie on the par-5 third at the TPC of Michigan when he dropped a sand-wedge within 10 feet of the cup. He stumbled to a bogey at the next when his second missed the green left in a bunker.
Morgan, who won this event in 1998, hit a pitching-wedge to 15 feet on No. 6 to set up his next birdie. He made the turn at 2-under when he rolled in a birdie putt from the fringe on the ninth.
'Well, I just kind of got off to a regular start,' said Morgan. 'Obviously I didn't anticipate this type of round at that point. I just kind of kept going. Really didn't shoot -- a couple under on the front maybe, and then kind of got started on the back.'
Around the turn, Morgan notched back-to-back birdies from the 11th to begin his move up the leaderboard.
Morgan, a doctor of Optometry, jumped into the lead with a strong late stretch. At the par-4 16th, he dropped a 9-iron within five feet for birdie.
Morgan knocked a 7-wood within 15 feet at the par-5 17th. He rolled in that eagle putt and then sank another 15-footer for birdie at the last to grab a two-stroke advantage after one round.
'I think as a player you kind of watch the scoreboard to kind of give you an idea of how receptive the golf course is,' Morgan said. 'So if you see some numbers up there, you just feel like maybe if I can play well, I can get in that same category and that's what happened.
'So I think it was something that sort of pushed me forward to try to keep doing better and actually it just happened coming down the stretch.'
The opening-round 64 tied the first-round record set last year by Hale Irwin. Morgan credited a new driver for his solid round.
'I changed drivers for the Senior Open but I just didn't play it very much,' said Morgan. 'I was hoping that maybe it would help my confidence and I seemed to drive it a little better today. You have to drive it in the fairway pretty much to do well.'
Doyle, the 2001 Senior Players winner, mixed three birdies and two bogeys over his first nine holes.
'Just one of those things, I felt I hit the ball very good on the front,' Doyle said. 'I kept putting myself in good spots. It seemed maybe just the way you hit certain shots.'
On the back side, he ran off back-to-back birdies from the 10th and again from the 16. He snuck a birdie in at the par-4 14th as well to grab a share of second.
'It seems we need to tighten things up, and make a determination that if someone shoots 4- or 5-under this week, they will win the tournament,' said Doyle. 'If you look at the PGA Senior, they had narrow, narrow fairways, but never got their green speeds up, because of all the rain. Then the Senior Open was a classic where the speed of the greens were probably running two feet more than these.'
Tewell ran off three straight birdies from the fifth, then notched three more birdies on the back side.
McCullough notched two birdies on the front nine before moving to minus-3 with a birdie on No. 12. He joined the tie for second by birdieing the final three holes of his round.
Kite used a strong back nine to shoot 66. He birdied the third, then made back-to-back birdies from the 10th. He eagled No. 13 to get to minus-5 then climbed to 6-under with a birdie at the 16th.
Gibson birdied the second then made it two straight with a birdie at the next. He dropped a shot at the sixth, but again made back-to-back birdies from the seventh to make the turn at 3-under.
The 55-year-old ran off four birdies in a row from the 14th to get to 7-under. He held first place by himself at the time, but dropped a shot at the last to fall into a share of second.
Fuzzy Zoeller, who has finished in the top 5 of the first two majors this season, fired a 5-under-par 67. He shares seventh place with television broadcaster Andy North, Tom Purtzer, Dana Quigley and Champions Tour rookie Craig Stadler.
Defending champion Stewart Ginn struggled to a 1-over-par 73. He is tied for 46th with nine other players.