Pornanong Phatlum held onto a share of the lead, but was joined at 9-under par by Anna Nordqvist and So Yeon Ryu at the Lorena Ochoa Invitational in Guadalajara, Mexico on Friday.
Nordqvist and Ryu, playing together, shot 5-under rounds of 68 at the Guadalajara Country Club on Sunday to reel in Phatlum, who followed up her opening-round 66 with a 3-under 69 on Friday.
"You know you just try to keep up and you know feed off of each other and give yourself a large change for birdies," Nordqvist said.
Phatlum is seeking her first official LPGA Tour victory.
"Today, I'm very happy with my score, but the putting not very good today," said Phatlum. "I just missed a few short putts."
Top-ranked Inbee Park and Americans Gerina Piller and Lexi Thompson are one shot off the lead heading into the weekend.
Suzann Pettersen and American Stacy Lewis, the Nos. 2 and 3-ranked players on the LPGA Tour, are in a group of four golfers at 6-under.
Taylor Collins, winner of this summer's "The Big Break," fell to the bottom of the 36-player field with a second-round score of 77 after posting a solid round of 74 on Thursday. At 7-over, she is two shots behind Spain's Beatriz Recari.