England's Shadoff grabs four-stroke LPGA Mediheal lead
Shadoff, who opened with a 64 to seize the first-day lead, stood on 11-under 133 after 36 holes at The Saticoy Club in Somis, west of Los Angeles.
"Playing really consistent and I feel really in control of my game," Shadoff said. "Just staying in my routine and hitting some good shots, rolling in the putts, so everything seems to be going well so far."
World number 93 Shadoff has not won a title in 12 LPGA seasons but was third at June's ShopRite Classic, her best result since a runner-up effort at the 2017 British Women's Open.
"I feel in control of my swing and how I'm hitting the ball," Shadoff said. "That definitely helps the confidence going into the weekend."
Shadoff, who led by two when the day began, birdied four of the first six holes, including the first two, the par-3 fourth and par-5 sixth before a bogey at seven.
She answered with a birdie at the par-3 ninth and responded to another bogey at the par-3 13th with a birdie at the par-5 14th before a last bogey at 17.
"A really nice start. Hit it really close on four, so 3-under through four holes is a very quick start," Shadoff said.
"Wind started to pick up a little bit on the back nine, which is a little bit tricky, but I played really well again and hopefully do the same at the weekend.
"Couple putts here and there just didn't fall. Overall I'm striking the ball well, hitting a lot of fairways, which is key out here. Putting it into the right parts of the greens, which definitely helps."
Reto, who won her first LPGA title in August at the Canadian Women's Open, birdied three of the first five holes on her way to shooting 70 to stand second on 137.
"I feel so good. I'm excited," Reto said. "It just boils down to being able to read those putts.
"If I put some extra time into that on the putting green I feel like I can give it a run tomorrow and Sunday for sure."
A third-place pack on 138 included China's Lin Xiyu and Liu Ruixin, Mexico's Gaby Lopez, France's Pauline Roussin, South Korea's Kang Hae-ji and Ji Eun-hee and Northern Ireland's Stephanie Meadow.
