A six-time winner on the Japan LPGA Tour, the 26-year-old Nishimura closed with a five-under 66 on Sunday (Monday AEST) for a three-day total of 21-under 192 at Pinnacle Country Club.
"I still can't believe it. I still shake my hand. But, yeah, I'm proud of myself and, yeah, I'm super happy," Nishimura said.
Leading by five shots entering the day after back-to-back 63s, Nishimura shot even-par 36 on the front nine while another Japanese player, Aki Iwai, made a charge with six birdies in seven holes to start the back nine.
Iwai shot a bogey-free 62 and tied for second with Thailand's Chanettee Wannasaen (65) and Korea's In Gee Chun (67).
"This week greens are so soft, especially today," Iwai said.
"So I was like, I need (to be) more aggressive. Like, yeah, I need to hit three, four yards past the pin usually."
Nishimura opened a three-shot lead over Iwai with an eagle on the par-5 14th after reaching the green with a fairway wood over water. She added two more birdies to shoot a five-under 30 on the back nine.
"I told myself to go aggressive until the very end and I went for it," she said.
Robyn Choi was the best-placed Australian, finishing in a tie for 51st at six under after rounds of 67, 71 and 69.
The only other Aussie to make the cut, Cassie Porter, tied for 64th at four under, following up her opening-round 67 with two 71s.
Lauren Coughlin, who set an LPGA record by opening with nine straight birdies to shoot a career-best 61 on Saturday, closed with a 66 and finished in a tie for 12th, eight shots back.
Lottie Woad, the seventh-ranked player in the world who helped Europe to victory over the US in the Solheim Cup two weeks ago in the Netherlands, withdrew with an illness. The English player was six shots off the lead after two rounds.
Two players made holes-in-one Sunday. Yealimi Noh aced the par-3 15th with a nine-iron from 136 yards, the first of her career.
Mao Saigo holed a seven-iron from 159 yards on the par-3 third, also her first ace.
- with AAP
