Naomi Osaka booked her ticket to the Toray Pan Pacific Open semifinals with a straight-sets win against Barbora Strycova of the Czech Republic on Friday.

Having quickly overpowered Slovakia's Dominika Cibulkova in the previous round, the U.S. Open champion had to dig deeper for her 6-3, 6-4 victory over the 25th-ranked Strycova in 1 hour, 31 minutes.

Osaka, the world No. 7, struggled to land her first serve early in each set at Arena Tachikawa Tachihi and gave her opponent a number of second chances with inaccurate shots. But in both sets, the local favorite eventually took control after finding her rhythm.

The 20-year-old Osaka said she had been prepared for a tough match against the eighth-seeded Strycova, having split their two past encounters on the WTA circuit.

The 20-year-old Osaka said she had been prepared for a tough match against the eighth-seeded Strycova, having split their two past encounters on the WTA circuit.

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

Osaka, who won the previous meeting between the pair in the opening round of Wimbledon last year, broke Strycova in the fourth game to go up 3-1, but the 32-year-old Czech broke back in the seventh game.

In the following game, third seed Osaka produced an array of dazzling returns to break her opponent once again before serving out the set in 42 minutes.

Having saved a break point in her opening service game of the second set, Osaka leveled the scores at 1-1 with her third ace of the match, a 190-kilometer-per-hour thunderbolt. She secured the crucial break in the fifth game to go up 3-2.

In the 10th game, Strycova saved one match point with a cracking forehand winner for deuce, before Osaka earned another match point with an ace. The favorite then made no mistake, sealing the win with an unreturnable serve.

Awaiting Osaka in the last four is Italy's Camila Giorgi, who led 5-3 in the first set against Belarusian Victoria Azarenka before the former No. 1 was forced to retire with an apparent thigh injury.

The unseeded Giorgi had reached the quarters with a 6-2, 2-6, 6-4 upset of top-seeded Dane Caroline Wozniacki in the round of 16 on Thursday.

Earlier in the main arena, 2017 quarterfinalist and former world No. 1 Karolina Pliskova of the Czech Republic survived a thriller against unseeded American Alison Riske.

After breezing through the first set, current world No. 8 Pliskova had to fight hard for her 6-1, 6-7(5), 7-6(4) victory over the tenacious Riske, who had reached the quarters by upsetting another former No. 1, sixth-seeded Spaniard Garbine Muguruza, in the last 16.

No. 4 seed Pliskova will face Donna Vekic in the semifinals after the 45th-ranked Croat upset sixth-seeded Frenchwoman Caroline Garcia 6-3, 6-4.