The Australian's total of 15-under 201 put her a shot ahead of compatriot and defending champion Katherine Hull (65) and South Korean Lee Bo-mee (63). Those three will be in Sunday's final group, pitching Hall of Famer Webb against the woman tipped to succeed her as Australia's dominant female player.
Webb was in a fairway bunker on the final hole but hit a 6-iron cleanly out of the sand about 140 meters, over a tree and to the green and sank the 15-foot putt.
Organizers say they are considering Lee's 63 a course record because they changed two tee boxes this year. But a 62 shot by Australian Kristie Smith in the tournament last year is the lowest score shot on the course before it was lengthened slightly.
Two South Koreans were tied for fourth after shooting 66s - Seo Hee-kyung and Ryu So-yeon - on a Royal Pines resort course softened by rain and made easier by the lift, clean and place rule used in the first round.
Former American amateur star Amanda Blumenherst, who led after the first two rounds, shot 70 Saturday and was tied for sixth, four off the lead and level with Taiwan's Yani Tseng (67).
Lim Ji-na, who shared the second-round lead with Blumenherst, shot 75 and was tied for 25th, nine shots behind Webb.
The six players who will make up the final two groups on Sunday accounted for 42 birdies and an eagle in the third round.
“I hit a great shot into the green there,” Webb said of her last-hole effort. “Anything on the green I was happy with. It was a great way to finish.
“If you told me I would shoot 64 today, I would have thought I'd have at least a two-stroke lead, but it will make it exciting for tomorrow.
“I'm just glad that I'm not three, four, five shots back going into tomorrow, and having to shoot a low one to have a chance.” Webb said her experience of winning six previous Masters titles will help her.
“I've made some good shots here before and great putts, and when you have those kinds of feelings, it makes you comfortable,” she said.
