Thompson meanwhile said she was pleased with her round after struggling through back pain. Im just going to go out here, have fun, be relaxed, just how I have been the last three days. I was so good at rolling the ball on the line.
Ko followed her opening-round 67 with a three-under 68 to hit the halfway point at Blythefield Country Club tied for 13th on seven under, just three shots off the pace set by Canada's Alena Sharp on 10 under, with four holes to play.
Sei Young Kim won the Meijer LPGA Classic on Sunday for her second victory of the year, beating Carlota Ciganda with a birdie on the first hole of a playoff.
Canadian youngster Brooke Henderson, who won the Women's PGA Championship last weekend for her maiden major, saw her challenge falter with a one-over-par 72.
"I just hit a lot of drivers and, if my game's on, it comes together", said Thompson, possibly in danger of missing the Olympics if the back problem lingers into the busy summer.
"It was going nicely but again I had bogeys starting from that hole (No. 13) and it's really wasteful", said Nomura, who is seeking her third win of the season. "All you can do is try the best you can and if the girls, like last week, if somebody plays good, you can't do much about it".
The 19-year-old New Zealander won the Kia Classic and the major ANA Inspiration in consecutive weeks this year in Southern California. She also won a year ago in Portland, Oregon. "It is tougher to win now, but I'll aim for the top five", she said.
Henderson played her final nine holes in 4 under. "I started out not as well as I would have liked, 1 over for the front nine, but I made a lot of birdies on the back and go 4 under, so overall it was a pretty steady day".
Michelle Wie shot a 67 to move into a tie for 28th at 5 under. "If I bounce back, yeah, it is a good work, so I did it well today".
Ariya Jutanugarn, third last week after winning her previous three starts, tied for 18th at 7 under after a 68.
Chun, who has not won a tournament since her breakthrough major victory at the US Women's Open a year ago, will look for a repeat of her solid iron play and putting in Sunday's final round.