Local favorite Kimiko Date Krumm has defeated Maria Sharapova 7-5, 3-6, 6-3 in the first round of Tokyo.
Sharapova led by a break and 40-0 at 3-2 in the third set but double faulted twice on her way to losing the game.
"If you give your opponent a chance they can get confident and take the momentum away," said Sharapova.
Date, who turns 40 on Tuesday, scored her second win over a former No. 1 since returning to the tour after a 12-year retirement. She defeated Dinara Safina twice earlier this year.
Safina lost 6-1, 5-7, 6-2 to Julia Goerges in Tokyo, while another former No. 1, Ana Ivanovic, defeated last week's Seoul champion Alisa Kleybanova 6-3, 6-2.