Issei Okamura defeated Jumpei Yamasaki 4-6 7-5 6-1 in the M25 Matsuyama 1 round of 32 on hard. The upset overturned the form book — Jumpei Yamasaki came in ranked #1025, leading the head-to-head 4–0. Issei Okamura claimed a first win over Jumpei Yamasaki in five meetings, narrowing the head-to-head to 4–1.
| Date | Tournament | Round | Surface | Winner | Score |
|---|---|---|---|---|---|
| 2025-04-22 | M15 Osaka 2 | R32 | hard | Jumpei Yamasaki | 6-3 6-4 |
| 2024-03-18 | M15 Nishi-Tokyo 1 | Q32 | hard | Jumpei Yamasaki | 6-2 6-1 |
| 2019-04-04 | M15 Kashiwa 1 | R16 | hard | Jumpei Yamasaki | 6-3 6-2 |
| 2016-09-05 | Thailand F3 | R16 | hard | Jumpei Yamasaki | 4-6 6-2 6-4 |
Averages over each player's completed matches in the selected window (5 vs 16). ATP, WTA & Challenger.
Each player's serve crossed against the other's return, converted to expected hold of serve. Recent completed matches, ATP, WTA & Challenger. About