The 34-year-old Pak, winner of five Major LPGA titles, shot a one-under-par 71 for a three-under-par 141 total on a windy day at a Locust Hill Country Club course lined with deep, unforgiving rough for the second women's Major of the year.
"This week you come out, you don't (have) very high expectation," Pak said. "Of course you really want it. My shoulder isn't any problem at all."
Pak is returning to action six weeks after suffering a partial tear to the labrum of her left shoulder in a fall down some stairs during a tournament in Mobile, Alabama.
"I'm very happy about the finish today, the round, because I got a couple of great up and downs from the rough and some great putts too," said the player who inspired a generation of South Koreans to excel at golf and make their way to the LPGA Tour.
"I'm playing really well actually. I'm really happy about it."
Tied one stroke behind Pak on 142 were 2010 US Open champion Paula Creamer, who shot level par 72, 2008 US Open winner Park In-bee of South Korea (70), 2010 Japan Women's Open winner Mika Miyazato (72) and Sandra Gal of Germany (71).
Six players were bunched another shot back at one-under-par 143, including former champion Suzann Pettersen of Norway.
Pak had three birdies, including back-to-back birdies at 16 and 17 (her seventh and eighth holes) and two bogeys as she took aim for her 26th LPGA win and first major triumph since the 2006 LPGA Championship.
After consecutive LPGA Championship romps by Yani Tseng last year and Cristie Kerr the year before, the Locust Hill fairways were narrowed and the rough grown thick and players have struggled to make par.
Along with the punishing wind, that added up to a tough challenge for the field. A total of 73 players made the cut, set at seven-over-par 151.
World number one Tseng, who won the title last year by 10 strokes with a 19-under-par total, made the cut without a stroke to spare after a three-over 75 that followed a 76.
"Before teeing off, we know it's a lot tougher because we could see the wind blowing," Pak said. "This golf course is a lot harder than the last couple of years.
"It's very difficult because if you miss the fairway, the next shot is from the rough. You have to use really smart thinking."
Creamer agreed but said she liked playing courses where par is a good score, such as Oakmont, where she won her US Open.
"I love this kind of golf," Creamer said. "This is to me what it's all about. It's hitting shots. It's hitting fades, hitting draws, knock downs, everything.
"It's not just trying to land it on a huge fairway. You have to golf your ball around out here and that's what you have to do at Oakmont and that's what you have to do at most Majors and this is right up there with it. This is difficult. This is a tough test of golf."