Inkster, who was the last player to successfully defend her title at DuPont Country Club in 1999-2000, fired a 5-under 66, while Rosales only managed a 1-under 70.
Tour rookie Reilley Rankin (67) and Soo-Yun Kang (68) are knotted in fourth place at 5-under-par 137.
Since Friday's play was washed out due to overnight rain and a forecast for more storms, the second round was contested on Saturday. The plan is for 36 holes on Sunday to determine the winner.
When physical conditioning comes into play, Sorenstam immediately rises from the pack. She spent Friday, and many other days, in the gym and may have an advantage over her closest competitors.
'For sure it won't hurt me,' said Sorenstam, who thinks the last time she played 36 holes in one day was at last year's Solheim Cup. 'I really don't mind. I shouldn't run out of gas.'
Sorenstam played even-par golf on the front side with nine pars, but really caught fire as soon as she made the turn. She knocked a sand-wedge to 3 feet to set up birdie at the 10th, then made it back-to-back birdies with a 10-footer at the par-5 11th.
The defending champion sank a 6-footer for birdie at the 12th. Sorenstam made it four birdies in a row at No. 13 when she hit a 5-iron to 25 feet at the 184-yard, par-3 hole.
'I wasn't really frustrated (on the front nine),' said Sorenstam, who defeated Grace Park, this year's Nabisco Championship winner, in playoff to win this title in 2003. 'On 10, it was a touchy little putt. After that, it started something really good.'
Sorenstam came up short with a 7-wood at the 465-yard, par-5 16th, but the Swede chipped 4 feet past the hole and rolled home the birdie putt.
At DuPont's closing hole, Sorenstam landed 40 feet from the hole with a 7-iron. She left herself with 7 feet for par after a horrible birdie putt and eventually missed the par-saver.
Sorenstam's lead was cut to one, but it's still important for the Hall of Famer to have the 36-hole lead. She was won 11 out of her last 12 with the 36-hole lead, including the last seven in a row.
'To be in contention in a major championship, that's what it's all about,' said Sorenstam, who has the career Grand Slam and six major titles to her credit. 'It doesn't seem like it's ever enough. I still want to win another one.'
Standing in her way is Inkster, a fellow Hall of Famer and two-time winner of this event herself. Inkster started on the back nine Saturday and tallied three birdies over her first nine holes.
At the fifth, Inkster played a 7-iron to 35 feet and holed the long birdie putt. She made it two in a row at the sixth when she wedged her approach to 2 feet.
Trouble loomed when Inkster was between clubs at the par-3 eighth. She elected to hit a hard 5-iron and sent the ball into the trees. Inkster punched out and chipped to 3 feet, where she saved bogey.
'I forgot to finish my back swing,' said Inkster, referring to the hiccup at eight. 'I made a great bogey there. It was my only bad swing of the day and I moved on.'
Inkster came up short of the green with her second at the par-5 ninth, but chipped to 6 feet. She ran home the birdie putt and put herself in position for victory No. 3.
'I love the golf course,' said Inkster. 'What you want to do is give yourself a chance to win the tournament and not knock yourself out on the first couple of days.'
Rosales was 1 over on her front nine, the back side at DuPont, but two long putts got her back on track. She drained a 30-foot birdie putt at the first and added a 20-footer for birdie two holes later.
That was all Rosales needed to get within one shot of the lead.
'I hung in there today and I liked the way I finished,' said Rosales, who earned her first LPGA Tour victory earlier this season at the Chick-fil-A Charity Championship. 'I stayed calm. It turned out pretty good today.'
Park shot a 1-under 70 and is tied for sixth place with Lorena Ochoa (67) and Pat Hurst (69). That group is knotted at 4-under-par 138.
Gloria Park (72), Stephanie Louden (68) and Shi Hyun Ahn (70) share ninth at minus-3.
The 36-hole cut fell at 4-over-par 146 and among the notable players who will not take place in Sunday's marathon are: reigning U.S. Women's Open champion Hilary Lunke (148), Kelli Kuehne (148) and former winner Chris Johnson (149).