World-ranked 85 Momoko Ueda took the sole lead after the second round of her home LPGA Japan Classic on Friday as new world number one Atthaya Thitikul moved ominously four shots behind.
Ueda shot a three-under-par 69 to move to 10-under overall at Seta Golf Course in western Shiga Prefecture, leading compatriots Miyu Yamashita and Ai Suzuki by one stroke.
Japanese players made up six of the top seven on the leaderboard as the LPGA tour returns to the country for the first time since 2019 because of the coronavirus pandemic.
Thai teenager Atthaya moved four shots off the lead after shooting a five-under 67 to take her to six-under overall and leave her well placed in her search for a third LPGA title of a stunning debut year.
The 19-year-old, who this week became the second-youngest world number one in women's golf history, said she played "a lot better than yesterday" after carding seven birdies and two bogeys.
"I still think this course is pretty challenging for us -- pretty narrow fairways and pretty slopey greens," she said.
"I think it depends on the day because today it's been my day."
Ueda had five birdies and two bogeys to break clear of the pack and upstage some bigger names in the field.
Further back, world number five Minjee Lee was in a tie for 24th, at three-under, after carding a second-round 69.
The Australian played with Japan's Mao Saigo and American Yealimi Noh and said she was impressed by the atmosphere generated by the home fans.
"Because I was playing a little earlier today, I didn't think there would be as big of a crowd," said Lee.
"But I played with another Japanese player and an American and we still had a pretty nice following, which was really nice."