Ji was 4 under at Locust Hill. Webb had a 68, matching Ji for the best round of the tournament.
Giulia Sergas, who shared the first-round but had a 76 on Friday in the wind-swept second round, moved back near the top with four birdies on the front nine and also finished with a 69. Sergas was tied at 2 under with Stacy Lewis, Suzann Pettersen and Inbee Park. Lewis, a two-time winner in her last three events, had a 70, Pettersen shot a 71, and Park had a 72.
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Paula Creamer was at or near the top for most of the day, but faltered at the end and finished with a 73. She was in a seven-way tie at 1 under.
Defending champion Yani Tseng had her best round of the three days, finishing with a 74 after a 76 and 75.
There were 24 players within four shots of the lead at the start of play. When the day ended, there were 13, including 2010 champion Cristie Kerr, within four shots of the top heading to the final round.
Over the first two days, only 28 players broke par and only six scored below 70. Webb added her name to the latter list, matching Ji for the best round of the tournament as the course played a lot easier than it had the first two days.
The start of play on Saturday was delayed for 2½ hours by rain, adding tension to the moment, but the predicted storms held off. That allowed the players, who went in threesomes off both the first and 10th tees, to finish without another delay.
Ji, who shot a 68 in the second round, was in a six-way tie two shots off the lead to start the day. She made four birdies and a bogey on the front nine and made the turn tied for the lead at 3 under.
Ji, who won here at the Wegmans LPGA in 2008, took sole possession of the top spot with a birdie at the par-5 11th hole. Creamer tied her at 4 under with a birdie at No. 12, but bogeyed the next hole and two of her final three to fall back.
Se Ri Pak, still not fully recovered from a left shoulder injury suffered in April, had been the picture of consistency the first two rounds, shooting 70 and 71, and began the day with a one-shot lead.
The magic was gone, though, on this day, and it was evident after her first swing.
Pak drove the right rough at the opening hole and was unable to get up and down, making bogey to fall out of the lead. She followed that by making three more bogeys before the turn and added another at No. 10 to fall to 2 over, six shots behind Ji. Pak finished with a 76 and was five shots back.
Creamer hit 10 of 14 fairways and reached 16 of 18 greens in regulation, but aside from a strong birdie to start the round, her putter deserted her all too often.
Creamer's long birdie putt attempt at the par-5 eighth hole broke nicely toward the cup but didn't have enough behind it, leaving her shaking her head in frustration at a missed opportunity. She also missed another short birdie at No. 10 despite perfect pace on the ball as it came to a stop an inch or two to the left of the cup and slid another just past the hole at the par-5 11th hole.
"It's pretty disappointing," Creamer said. "I kind of got in my way with my putter."
Lewis missed a short birdie putt at No. 10 and did so again at the 12th hole, her 5-foot putt curling left of the hole as she, too, shook her head in dismay. A bogey at 13 dropped her back to 1 under, but a birdie at 17 put her in a position she relishes.
"It's a lot easier coming from behind," said Lewis, the top-ranked American. "It's so hard to keep your focus. I almost like being at the back and kind of coming up and surprising someone."
Just how difficult were the conditions at Locust Hill over the first two rounds? For Tseng, they were insurmountable.
In winning the LPGA Championship by 10 shots a year ago, she set a tournament record for most birdies with 26 and came in to defend her title here leading the tour in birdie average (4.64). She managed only five over the first 42 holes to go with a whopping 14 bogeys as the narrow course proved the toughest of challenges with its extra-thick rough and swirling winds.