Karlsson is the only player to finish in the top 10 in all three majors this year
Sweden's Robert Karlsson and India's Jeev Milkha Singh shared a one-shot lead after the first day of the USPGA.
The pair shot two-under 68s before a bad weather delay of over an hour derailed some of the late starters.
Argentine Andres Romero reached two under after 16 holes before darkness then stopped play, while Sergio Garcia (69) was in the group on one under.
Phil Mickelson shot 70, Padraig Harrington took 71 but Lee Westwood amassed a round of 77 at Oakland Hills.
Also in the clubhouse on one under were Americans Ken Duke, Billy Mayfair and Sean O'Hair, while countrymen Anthony Kim and Ryan Moore, Australia's Rod Pampling and Argentina's former US Open champion Angel Cabrera all carded level-par 70s.
Eighteen players out of the 156-man field were still to complete their first rounds and will resume at 0800 local time (1300 BST) on Friday with the second round starting as scheduled at 0730 (1130).
For a long time I thought I could get better by looking at things on the outside, techniques and whatever, instead of looking inside
Karlsson, 38, began with a double bogey on the tough 7,395-yard layout but rattled in five birdies in the next seven holes and reached four under after 11 before handing two shots back before the finish.
The 6ft 5in Swede, who is ranked 22nd in the world, is the only player to record top-10 finishes in each of the first three majors in 2008.
"I now feel comfortable playing in the big events where as before I had a bit of a problem," he said.
"I was very nervous. I always had to prove myself, that I was good enough, and I don't feel that way at all this year."
This season Karlsson has tied for eighth place at the Masters, shared fourth spot in the US Open and finished joint seventh at the Open.
"For a long time I thought I could get better by looking at things on the outside, techniques and whatever, instead of looking inside, looking at what I have," he said.
"I worked a lot on my relationship with my kids, working more on how I relate to them, and my relationship with my wife."
Singh, the son of an Olympic sprinter who competed for India in 1960, eagled the second and mixed three bogeys with three birdies.
"The most important thing to learn at a major championship is to stay patient, because they are going to test your patience," he said.
"You're going to get some bad breaks, you're going to hit some shots in the rough and you sometimes can't get it out."
The 28-year-old Garcia began at the 10th on Thursday morning and traded one birdie and one bogey before a solitary birdie among eight pars on the way back.
"I scrambled nicely," said the Spaniard, who needs a good result to qualify automatically for the European Ryder Cup team.
"I putted good. I chipped good. I hit a lot of good shots into the greens and one-under-par on this course, I'm thrilled with it."
Double Open champion Harrington got off to a blistering start and birdied his first three holes, but after missing a short putt for another birdie at the 4th he struggled with five bogeys and a birdie from the 7th.
"It felt like I played a lot better than 71," he said. "I really struggled on the greens and I had a number of putts that the hole lipped out on, so it made me feel like I was putting a lot worse than I was maybe.
"The score is good. It's only the first round. This is all you want in the first round; you want to keep yourself in there. It's all about staying patient for the first three days and I need to get my head around that."
Back on one over in the clubhouse was a large group that also included Ernie Els, Jim Furyk and Henrik Stenson, while on two over were the likes of England's Paul Casey and Retief Goosen.
South African Goosen, a two-time US Open winner, was one of the biggest victims of the suspension for thunderstorms.
He reached four under to lead by two at one stage but was back to a share of the lead with Jonathan Byrd at two under when the hooter sounded to call off play.
On the resumption following a delay of one hour 25 minutes, Goosen added four more bogeys for a 72, while Byrd was one over after 15 holes before darkness brought an end to proceedings.
England's Justin Rose ended with a 73, Ian Poulter and Graeme McDowell took 74, Darren Clarke finished with 75 and Colin Montgomerie and Vijay Singh were one shot further back.
American Kenny Perry withdrew after his round of 79 with an eye injury.