Webb retained a piece of the lead she held Thursday with In-Kyung Kim by shooting 6-under 66, while Gustafson shot a sizzling 7-under 65 with a bogey-free round that included seven birdies -- but none on the final four holes, which allowed Webb to share the top spot.
"I was feeling a little more comfortable over the ball (today)," Gustafson said. "I think it made a difference."
"The scores are pretty low out there, and the greens are very receptive," Webb noted. "It's a bit of a ball striker's course."
Teresa Lu (67) and Inbee Park (68) are tied for third place at 9-under 135.
Three others share fifth at 8-under 136, while the tournament's annual host, world No. 2 Annika Sorenstam, is tied for eighth at 6-under 138 despite shooting 6-under 66 Friday.