Thailand's Phatlum and Icher of France carded eight birdies apiece but both bogeyed the par-four 11th.
America's Katie Futcher was a shot behind them, ahead of a group of three players on five under.
"It was a good round, a good sensation," Icher told www.lpga.com. "I love that course. It's just flat and enjoyable so it was cool."
England's Karen Stupples, the only British entrant, was two over to sit joint 24th out of 30.