Reto carded eight birdies to easily offset her lone bogey, which came on her second hole of the day on Thursday at Evian Resort Golf Club in France.
"This golf course is so (much) fun and the views and everything, it's just awesome to be here," she said.Â
"You never know out here with this golf course. Anything is possible. You just got to give yourself the best opportunity to make par and birdie."
Reto holds a two-shot lead over a group of four players that includes former world No.1 Lydia Ko of New Zealand, France's top-ranked player Celine Boutier, American Alison Lee and Thailand's Wichanee Meechai.
Defending champion Brooke Henderson shot a two under 69 on Thursday and is in a deep group tied for 19th that also includes LPGA Tour rookie star Rose Zhang and Linn Grant, who won the Dana Open two weeks ago.
Current World No.1 Jin Young Ko of South Korea and second-ranked Nelly Korda are six shots off Reto's lead after opening with one under 70s.
The Evian Championship is the fifth and final major of the year.
"If I can just go out and give myself the best chance possible, good club selection, know where to be on the green, just know when the putts are downhill, uphill, so all that stuff to keep my brain occupied with good information," Reto said of her mindset entering the second round.
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