Grace Park fired a 9-under 61 to climb into a share of second place at 14-under-par 196. She is joined there by Laura Davies, who shot a 66. First-round leader Jung Yeon Lee is one stroke behind that at minus-13 after a 3-under 67.
Stupples opened her round with a birdie on the par-3 second after she dropped a 7-iron within five feet. She then two-putted for birdie at the par-5 sixth to get to 13-under.
The 30-year-old then birdied the par-4 eighth for the third straight day. Later, she birdied the 13th, again for the third day in a row, and moved to minus-16 when she sank a 20-foot birdie putt at the very next hole.
Stupples, however, stumbled to a bogey at the last as she failed to get up-and-down from a green-side bunker to hold a one-stroke lead heading to the final round.
'I really hit the ball very well today,' said Stupples, whose best finish on tour is a tie for third at the 2002 Safeway Classic. 'I really don't think I made the most out of how I hit it today. I could have made a few more putts.'
Stupples is well aware of the fact that this event frequently yields low scores, so it will take a strong final round to hold on for the win.
'I know that you've got to be looking at at least 20-under for the winner,' said Stupples. 'I honestly feel like there's so many opportunities that you really have to step up and take them.'
Park, who turned 25-years old last week, began with three straight pars before a birdie on the par-4 fourth. She picked up another birdie at the sixth on the Dell Urich course.
The Korean dropped in back-to-back birdies from the eighth to get to minus-9. Around the turn, she ran off three consecutive birdies from the 13th to continue her climb up the leaderboard.
Park closed her round in style as she birdied each of the last two holes for a round of 61, which is one stroke off the course record. She hasn't bogeyed a hole since No. 11, which was her second hole, during the second round.
'I started a little slow with three pars on the first three holes,' Park said. 'I missed a couple of good opportunities in the first couple of holes, but then the birdies came.'
Davies stumbled out of the gate with a bogey at the second. The Englishwoman quickly rebounded with back-to-back birdies from the third and then picked up another birdie at the sixth.
She sank the fourth birdie of her round at the eighth to move to 13 under. On the back nine, Davies birdied the 13th, but stumbled to a bogey at the very next hole. She grabbed a share of second place when she converted a birdie try at the 16th.
'The slow start was a bit frustrating this morning, and then it played very slow early on,' said Davies. 'It was not a great round, but I still have a chance tomorrow, which is what you always want to do on Saturday.'
Stacy Prammanasudh, who earned her tour card as the leading money winner on the Futures Tour in 2003, fired a 7-under 63. She stands alone in fifth place at 12-under-par 198. Seol An Jeon, another LPGA Tour rookie, is one stroke further back at minus-11.
Aree Song, yet another rookie, is in seventh place at 10-under-par 200 after a 5-under 65. Se Ri Pak used a 64 to climb into eighth place at minus-9.