The Thai, seeking her first win on the LPGA Tour, shot a three-under 69 for a nine-under-par 135 after playing 36 holes at the Guadalajara Country Club.
"Today, I'm very happy with my score, but the putting was not very good today. I just missed a few short putts," Phatlum said.
Nordqvist and Ryu, who played together in the penultimate group, both hit five-under 67s to move alongside Phatlum.
"It's just a fun atmosphere and we played with Jenny Shin today and she was solid pretty much the whole round," Nordqvist said. "You just try to keep up and you know feed off of each other and give yourself a lot of chances for birdies."
World number one Inbee Park was a shot back after shooting a second round 68 with Lexi Thompson (64) and Gerina Piller (65) joining the Korean in fourth.
Lizette Salas (67) and 2010 champion I.K. Kim (67) were seventh while Suzann Pettersen (68) shared ninth with Karine Icher (68) Stacy Lewis (66) and Nordqvist's playing partner Shin (69).
Defending champion Cristie Kerr was alone in 27th place after hitting a five-under 67 to haul herself back to even par for the championship.
The tournament is hosted by former pro and Guadalajara native Lorena Ochoa, who held the position of world number one for over three years during a career that spanned from 2003 to 2010.
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- Jenny Shin