Osaka into Tokyo tennis semi-finals

Naomi Osaka's good run of form has continued, the US Open champion moving through to the last four in Tokyo.

Naomi Osaka beat Barbora Strycova to move through to the semi-finals of the Pan Pacific Open. (AAP)

US Open champion Naomi Osaka advanced to the semi-finals of the Pan Pacific Open by beating Barbora Strycova 6-3 6-4 on Friday.

The third-seeded Osaka converted all three of her break points and won 80 per cent of points on her first serve.

Osaka became the first Japanese player to win a grand slam singles title when she upset 23-time major champion Serena Williams in New York.

"This was my third time playing her, and each time it was very close, so I knew she was very good," said the third-seeded Osaka,.

Osaka will next face Camila Giorgi, who advancd after Victoria Azarenka retired from their match in the first set. Giorgi was leading 5-3 when the match ended.

Also, fourth-seeded Karolina Pliskova defeated Alison Riske 6-1 6-7 (7-5) 7-6 (7-4). She will next play Donna Vekic, who beat second-seeded Caroline Garcia 6-3 6-4.