Naomi Osaka raised her game when she needed to, and it landed her a spot in the semifinals of the Pan Pacific Open.

The US Open champion, who upset Serena Williams in the final in New York at the beginning of the month, won her ninth straight match Friday, beating Barbora Strycova 6-3, 6-4.

"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, the first Japanese player to win a Grand Slam singles title.

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

The 20-year-old Osaka converted all three of her break points and won 80 percent of points on her first serve.

She broke the eighth-seeded Strycova to take a 3-1 lead in the first set but was soon broken back. Leading 4-3, that's when Osaka took control. She immediately earned two break points, and Strycova double-faulted on the first one to put Osaka ahead 5-3.

In the second set, double-faults again cost Strycova, who was broken again at 2-2.

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

Youzhny calls it a day

ST PETERSBURG: Top-seeded Dominic Thiem reached the quarterfinals of the St Petersburg Open with a 7-6 (4), 6-4 win over Jan-Lennard Struff on Thursday.

The Austrian player's next opponent will be eighth-seeded Daniil Medvedev of Russia, who prevailed against Mikhail Kukushkin 6-3, 6-4.

Russian veteran Mikhail Youzhny retired from tennis after losing 7-6 (6), 3-6, 6-3 to fifth-seeded Roberto Bautista Agut of Spain.

The 36-year-old Youzhny won 10 tour titles and reached a career-high ranking of eighth in 2008.

Defending champion Damir Dzumhur of Bosnia progressed by beating Guido Pella 6-4, 6-4.

Dzumhur landed 70 percent of his first serves as he set up a meeting with Stan Wawrinka.

Nineteen-year-old Canadian Denis Shapovalov defeated Matteo Berrettini of Italy 7-6 (6), 4-6, 6-0. He next plays Martin Klizan.

Ostapenko loses

SEOUL: Ekaterina Alexandrova broke top-seeded Jelena Ostapenko's serve five times on Friday to beat the defending champion 6-3, 6-2 in a rain-delayed match at the Korea Open.

In other quarterfinals, Kiki Bertens beat Evgeniya Rodina 3-6, 6-3, 6-0 and AjlaTomljanovic defeated Mandy Minella 6-2, 4-6, 7-5.