Maggert took advantage of teeing off in the first group with calm conditions for the first seven holes, and finished with four birdies, an eagle and two bogeys. Merrick matched his best round of the year with six birdies and two bogeys, the last on No. 18 when his tee shot went into the water to drop him back into a tie for the lead.
"Had a loose shot there on 18, but you know ... can't hit every shot perfect out there," Merrick said. "So, I was really happy with today."
S.Y. Noh, Arjun Atwal, Jeff Overton and J.J. Henry were a stroke back.
McIlroy, coming off three straight cuts worldwide, played alongside Harrington and Graeme McDowell in an all-Irish threesome. McIlroy and McDowell are from Northern Ireland, and Harrington from Ireland.
The company and extra work on his game had McIlroy pretty happy Thursday despite two bogeys.
"I felt like that was about the best round of golf I played in a while. It could have been a lot better. Middle of the fairway, 3-under par have a 9-iron into the green par 5," McIlroy said of an approach shot that hit the water next to the green on No. 3 for his first bogey. "The scoring doesn't look too good. Off to a decent start definitely, 2 under is a decent start. Definitely so. Lot of good signs out there."
Golfers are used to playing through wind and muggy heat in Memphis. A front that kept the temperature a very comfortable 82 Thursday brought winds in from the north, and that created challenges most of the day when many expected good scoring conditions at the 7,239-yard TPC Southwind course.
"It's gusting a bit, but it's definitely from an unusual direction," Harrington said.
"And anybody who has played over the past number of years, it's playing a different golf course. On 9 there, I'm hitting driver off the tee. I normally hit 5-wood off the tee. There's a lot of holes to have changed like that out there. Some are much tougher, and some are a little easier."
Maggert is playing his 16th event this year after having shoulder surgery last June and earned his way back on tour at the qualifying tournament. He tied for 13th at the Sony Open and fifth in the Humana Challenge in January but has missed nine cuts.
He took a week off after missing the cut at Colonial and came to Memphis, where he won in 2006, even though his mother is recovering from surgery for breast cancer in Houston.
He matched his best opening round teeing off on No. 10. He had four birdies on his front nine, including on Nos. 14, 15 and 16 in making the turn at 4 under. He was the only player to reach 6 under, getting there when he eagled the par-5 third after hitting his second from 223 yards to 8 feet.
"I got off to a really good start," Maggert said.
Maggert bogeyed Nos. 5 and 8 to fall back to 4 under. He wasn't happy with his bogey at No. 8 after his ball on the par 3 stuck in mud in the fairway 48 yards shy of the pin. He called over two rules officials hoping for relief on a course that had dried out fairly well from heavy rains Monday considering the mud hadn't been there in Wednesday's pro-am.
"I felt like I was justified, but the powers that be thought that I was not," Maggert said.
Merrick tied for 11th here a year ago, and he matched the score he opened with in 2011 despite not getting a chance at a practice round on the course because he was at a U.S. Open qualifier pushed to Tuesday by weather Monday. He birdied three of his first four holes and birdied three of five on the back nine, his last taking advantage of hitting with the wind to get on the green at the par 5 in two before two-putting from 60 feet for birdie.
This is just the second time Merrick has had at least a share of the lead after 18 holes. He had a piece of the lead at the 2008 Mayakoba Golf Classic and wound up tied for third. Merrick, whose career best finish was second at the 2009 Bob Hope Classic, said he is looking at the leaderboard but isn't looking ahead.
"I'm really happy with where I am today," Merrick said.
Divots: Maggert is playing in Memphis for the 13th time in his career. This is the 10th time he has held at least a share of the 18-hole lead, the first since 2007 U.S. Bank in Milwaukee where he finished tied for fifth. ... Harrison Frazar, who won his first career title here a year ago, opened defense of his title with an 80. He's at risk of being only the third defending champ to miss the cut in Memphis since 2000. ... Luke Guthrie, a senior from the University of Illinois, shot a 69 in his tour debut after turning pro Tuesday. ... Garth Mulroy withdrew with a back injury after shooting 42 through nine holes.