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.
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