Phatlum followed her opening-day 64 with a 66 at the Columbia Edgewater Country Club and her 14-under 130 total was good for a one-shot advantage over Suzann Pettersen.
Pettersen turned in the low round of the tournament, a 9-under 63 that vaulted her from a tie for 20th into second place.
Phatlum has one victory in her five years on the LPGA Tour and her showing this week comes as a surprise based on recent results. She finished tied for 51st at the Canadian Women's Open last week and prior to that missed the cut at the Women's British Open.
Celebrity Breakups and divorces of 2014 [PHOTOS]