Tomoko Kusakabe shot her second straight 68 on Saturday to take sole possession of the lead after two rounds at the NEC Karuizawa 72.
Kusakabe, whose best finish was a second place in 2007, took a one-stroke lead over Yuri Fudo, who matched her 68, and overnight co-leader Yang Eun Bi of South Korea. Junko Omote, who shared the first-round lead with Kusakabe and Yang, shot an even-par 72 to fall four strokes off the pace.
Saturday marked the first time since her tour debut in 2004 that Kusakabe had finished a round all alone at the top of the leaderboard.
"I've mostly been missing the cuts, so I'm a bit surprised," said the 32-year-old, who had reached the final round just once in her last 10 tournaments and opted this week for a new putting grip.
"All of a sudden, I've begun putting well," she said. "I'm not getting younger, so I'd better make the most of this chance."