Four-time winner and defending champion Se Ri Park shot a 5-under 66 and is tied for second place with U.S. Women's Open winner Meg Mallon.
Brandie Burton, Catriona Matthew, Yu Ping Lin, Marisa Baena and Angela Jerman are knotted in fourth place at 4-under-par 67 at Highland Meadows Golf Club.
Stupples wasted little time in breaking into red figures. She knocked a 7-iron to 20 feet at the first to set up birdie, then rolled in a 6-footer for birdie at the third.
A pair of 9-iron approaches led to Stupples' next birdies. At the seventh, she sank a 15-footer, then closed her front side with a 6-foot birdie putt at nine.
Stupples missed the fairway at the 12th, then missed the green with a 7-iron. She holed a putt from off the green to reach five-under for the championship.
She played another loose 7-iron at the 400-yard 16th, missing the green left. Stupples chipped her third to 2 feet, but missed the par-saving putt. She closed with a pair of tap-in birdies at the par-5 closing holes at Highland Meadows to move clear of Pak and Mallon.
Stupples racked up her second win of the year last Sunday, but her first triumph in a major championship. She never considered withdrawing from this week and splurged with an upgrade in seating on the trip back to the United States. But Stupples is still basking in her accomplishment from a week prior.
'I am still on a high. I keep having to pinch myself,' said Stupples, whose tournament total of 19-under-par 269 at the Women's British Open tied the tournament record on a par-72 course. 'The way I played last week gave me a lot of confidence in how I managed myself around the golf course.'
Pak, who has six top-10s in the last six years at this event, recorded three birdies in her first seven holes, all from longer than 20 feet. She hit a pitching-wedge to 14 feet to set up another birdie at the ninth.
Pak played a 7-iron to three feet for a birdie at the 12th. Trouble loomed for this year's Michelob Ultra Open champion. Pak hit a nice sand-wedge to 10 feet at the 13th, but that was for par and the 22-time winner on tour missed the putt.
She closed her round with an eight-iron to 13 feet at the last. Pak stroked home the putt to get back within one of Stupples.
'This golf course forgives a lot,' said Pak. 'For some reason, it is fun to play this golf course. I know these greens really well. There are a lot of birdies out there.'
Mallon, who came from behind to win the U.S. Women's Open last month, opened on the back nine and got off to an amazing start. She birdied three holes in a row from the 11th, including a pair of 5-footers at 11 and 13.
She added another birdie at the 15th when she knocked an 11-wood to 15 feet. Mallon collected her fifth birdie on the first nine with another 15-foot putt at the par-five 17th.
Mallon, who won the Canadian Women's Open the week after her major triumph, birdied No. 1, but dropped a shot at the fourth when she missed a 4-footer. She parred out to join Pak in second.
'I got off to a terrific start today,' said Mallon. 'I am very happy with 5 under on the first day.'
Jeanne-Marie Busuttil, Amy Fruhwirth, Emilee Klein and Candy Hannemann are tied for ninth at minus-3.
Karrie Webb, Beth Daniel, Jennifer Rosales and Lorena Ochoa are part of a group in 13th place at 2-under-par 69.