The 22-year-old Masson made three birdies in the front nine and three on the back to hold off a late charge from Yani Tseng. The top-ranked Taiwenese player closed within a shot at one stage after picking up five strokes in as many holes, including an eagle.
"I was quite nervous at the start," said Masson, who has totaled 15-under 201. "I hit a poor tee shot at the first and then three-putted from 40 feet, but I settled down with a birdie at the next when I hit my 5-iron to 7 feet and holed it.
"I hit some really good shot out there today and really enjoyed it. The wind was a bit stronger and there were some tough pin positions."
Masson birdied No. 5 with a lay-up to 10 feet and the long No. 6 with an approach to 30 feet and two putts. At No. 11, she holed from 7 feet, then got up and down from a greenside bunker at the long 14th for another birdie.
Her last was at No. 17 but she dropped a shot at the next after hitting into another greenside bunker.
Masson will play the final round Sunday with Tseng, the 22-year-old prodigy who already has won four majors.
"It should be really enjoyable. Yani seems very nice and I'm sure I'll learn a lot playing alongside her," Masson said.
For Tseng it also will be a new experience.
"I have never seen her play," she said.
The American challenge never materialized. Lang (69) rose from a share of 18th place but is one stroke worse off than at the start of the day. Paula Creamer (71) is at 6 under, and Brittany Lincicome (76) is 13 back after only being five behind overnight.
First-round leader Meena Lee, tied for second overnight, fell out of contention with an 80 to sit at 2 under.