Shon had a 7-under 29 on her final nine, the front nine at Blythefield Country Club. The former Princeton star played the five par-5 holes in 5 under with an eagle on No. 8.
''Honestly, going into the round, I was thinking maybe the front nine was kind of going to be the tougher nine,'' Shon said. ''So I just was pacing myself, trying to stay in the moment and hit every shot the best as I could. I don't know, the back side, my putter started getting hot.''
Born in South Korea, the 26-year-old American is winless in four seasons on the LPGA Tour. She began the final-nine run with a birdie on the par-5 first, birdied the par-3 fourth, par-5 fifth and par-7 seventh, eagled No. 8 and closed with a birdie on the par-4 ninth.
''I haven't liked this golf course, but to be honest, it's playing a little bit different than it has in the past,'' Shon said. ''A couple of the tee boxes have changed and the course itself. The weather's just perfect, which is also something we're not used to all the time so far this year.''
The sixth-ranked Ryu birdied four of the par 5s in a bogey-free round.
''It's more about the putting instead of score,'' Ryu said. ''Obviously, when you're putting really well you have a chance for a really low round, but I just really wanted to have like confidence on the putting green. Like I don't think I have enough confidence on the putting green, that's why I always struggle. So from now on, hopefully, I can get fully confident when I'm putting.''
She winless this season after taking the major ANA Inspiration and Walmart NW Arkansas Championship last year. She also won the 2011 U.S. Women's Open.
''Like to be honest, if I talk about this season, I'm not really fully satisfied with it,'' Ryu said. ''Hopefully, this week I'm going to start playing really well and hopefully I can win this tournament.''
''I've had a really rough start to the year,'' Nordqvist said. ''Haven't really felt like myself, but just trying to make a few changes the last couple weeks and get on a better bit of a roll. I know good golf is ahead of me, but it's definitely been frustrating.''
''The weather couldn't have been any better for us out here,'' Thompson said. ''The course is in great shape. The last few days it's actually been pretty windy out here, but today there was like nothing.''
Ariya Jutanugarn, making her first start since winning the U.S. Women's Open, matched defending champion Brooke Henderson, Michelle Wie and Lydia Ko at 69. Annie Park, the ShopRite LPGA Classic winner last week in New Jersey, had a 76.