"I really felt like we had to take advantage of the conditions," McIlroy said. "I'm expecting this to be the best day of the week."
Thunderstorms were possible later Thursday and throughout the tournament, but skies were mostly clear when the first groups teed off. McIlroy, who won the U.S. Open last year, is trying to break an unusual pattern in which the last 16 majors have been won by 16 different players. He tied for 40th at this year's Masters, missed the cut at the U.S. Open and tied for 60th at the British Open.
McIlroy will be a contender on Kiawah Island's Ocean Course if he can build on the steady, bogey-free first round. McIlroy looked like golf's next big star after his U.S. Open win at Congressional in 2011, but expectations were tamer when he arrived at the PGA Championship for this year's final major. He finished tied for fifth last weekend at Firestone, perhaps a sign his game was coming around.
"I was working very hard on technical stuff for the last few weeks, and then a slight mental adjustment, as well," McIlroy said. "I felt like I was maybe hitting too many balls on the range. I just needed to go out there and play a bit more on the course and see shots."
Pettersson's 66 left him a stroke ahead of McIlroy and Gary Woodland (67). John Daly and defending champion Keegan Bradley both shot 68 along with Geoff Ogilvy.
Joost Luiten of the Netherlands was threatening the tournament's 18-hole scoring record at 8 under through 14 holes, but he bogeyed his last four to end up at 68.
Bradley, playing in a group with Woods, began in impressive fashion with a birdie and an eagle on his first two holes. After starting the round on No. 10, Bradley made a 25-foot putt for eagle on the par-5 11th, while Woods had to scramble for par after a wayward drive.
Woods bounced back with a birdie on No. 12, bogeyed Nos. 13 and 14 and made a birdie at No. 15.
"I'm playing with Keegan and he's 3 under through two, and you look up on the board, some guys 4 under through six, a bunch of guys 3 under through five, a couple of them were 3 under through three," Woods said. "So it's one of those days where everyone's going to shoot 6, 7, 8 under par, but the wind kicked up a little bit and it changed things quite a bit."
Woods, trying for his 15th major championship and first since 2008, looked frustrated when his tee shot missed the green to the left on the par-3 17th. But he saved par and then birdied Nos. 18, 1 and 2 to move back in the right direction. He ended up with six birdies and three bogeys.
"Anything in the 60s is going to be a good start in a major championship, and I'm right there," Woods said.
Bradley rallied to beat Jason Dufner in a playoff at this tournament last year, and he's coming off a victory at Firestone.
The 26-year-old Luiten, of the Netherlands, has one career win on the European Tour. A solid start Thursday became a great one when he eagled the par-5 16th. With four holes remaining, he had a chance to make a run at the tournament's 18-hole record of 63, but that fell apart quickly when he finished with a string of bogeys on Nos. 6-9.
Pettersson is back in South Carolina after winning the RBC Heritage there in April. He was second in the Houston Open two weeks before that, and finished tied for second at the Sony Open in January.
"I knew early on because there was no wind the first few holes, I knew it was gettable today," Pettersson said. "That's why it was important to shoot a good one today. I didn't quite see a 66 out there, but it was important to shoot under par today for sure."
Carl Pettersson -6
Gary Woodland -5
Rory McIlroy -5
G. Fernandez-Castano -5
Alex Noren -5
Tiger Woods -3
Phil Mickelson +1