Mayumi Yamamoto defeated Seiko Okamoto 6-4 6-1 in the Karuizawa 25K round of 32 on carpet. The upset overturned the form book — Seiko Okamoto came in leading the head-to-head 3–1. Mayumi Yamamoto narrowed the head-to-head to 3–2.
| Date | Tournament | Round | Surface | Winner | Score |
|---|---|---|---|---|---|
| 2007-09-17 | Tsukuba 25K | R16 | hard | Seiko Okamoto | 6-3 7-6(3) |
| 2006-07-10 | Miyazaki 25K | R16 | carpet | Seiko Okamoto | 6-3 6-0 |
| 2005-05-23 | Nagano 25K | F | carpet | Mayumi Yamamoto | 6-2 6-2 |
| 2004-05-17 | Beijing 25K | SF | hard | Seiko Okamoto | 6-3 7-5 |