Thitikul, a former world No.1, who entered the day nursing a one-stroke lead in Maineville, is seeking her first individual LPGA win in two years.
The 21-year-old from Thailand started her day hot with birdies on two of her first three holes (one and three) and sprinkled in birdies on holes eight, 11 and 15 before eventually suffering bogeys on back-to-back holes on 16 and 17.
She closed her day with a rally to grab a birdie on the par-5 18th, shaking off the challenge of the heat to sit on 16-under par.
"I think stay(ing) hydrated is the most important here," Thitikul said.Â
"And then you can see all the umbrella, which is the sun -- it's really killing us this week.
"But it's good. It's good. I like it more than the cold weather."
New Zealand's Lydia Ko, who Thitikul paired with, remains her closest pursuer at 14-under after her three-under 69.
Ko's round was steady after a bogey to open on hole one. From there, she scored birdies on the eighth, 10th, 14th and 18th.
Ko, who won Olympic gold in Paris and the AIG Women's Open also recently clinched enough career points to be inducted into the LPGA Hall of Fame.
China's Yan Liu fired a four-under 68 to earn third place honors after three rounds. Her day ran the gamut, including a double bogey on No. 3 and an eagle on No. 11. She balanced an additional bogey with five birdies.
South Korea's Haeran Ryu (70) and Switzerland's Albane Valenzuela (68) round out the top five at 12 under after 54 holes.
Five golfers are tied for sixth five shots behind the leader: Lindy Duncan (68), South Korea's Hyo Joon Jang (67), Australia's Stephanie Kyriacou (69), Thailand's Jasmine Suwannapura (69) and the Philippines' Bianca Pagdanganan (69).
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