Pettersson took advantage of the relatively calm conditions that greeted the players who teed off in the morning. The afternoon wave, on the other hand, battled much stiffer breezes and more are on the way.

Pettersson, who is bidding to become the first Swede to win a major championship, will take a one-stroke advantage over Rory McIlroy, Gary Woodland, Alexander Noren and Gonzalo Fernandez-Castano into the second round.

"I think I saw the first leaderboard around No. 3 or 4, and there was a bunch of guys 2 , 3 under, so I knew it was scorable," Petterson 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."

That’s because thunderstorms are expected overnight as a front begins to push toward the coast. More rain is possible Friday morning, and there is a 60 percent chance of storms in the afternoon when the winds are expected to gust over 30 mph. And Saturday is expected to be a carbon copy of the second round.

Holland’s Joost Luiten actually got to 8 under through 14 holes on Thursday but bogeys on his last four cut that score in half. He is among a big group tied at 4 under that also includes a quartet of major winners in defending champion Keegan Bradley, 1991 PGA winner John Daly and former U.S. Open champs Geoff Ogilvy and Graeme McDowell. Adam Scott, who nearly won the British Open last month, also shot 68.

Tiger Woods relied on a string of three straight birdies as he made the turn to come in with a 69. He’s among 11 players tied at 3 under, a group that also includes former British Open champ Ben Curtis and Japan’s Ryo Ishikawa.

British Open champion Ernie Els shot even par while Bubba Watson, who won the Masters, opened with a 73. U.S. Open champion Webb Simpson, who was playing for the first time in a month, was unable to shake off the rust and shot 79.