Karolina Pliskova of the Czech Republic advanced to the Bank of the West Classic semifinals Friday, beating Croatia's Ajla Tomljanovic 6-2, 6-4.
The 11th-ranked Pliskova will make her debut in the top 10 when the new rankings are released Monday.
Only world No. 1 Serena Williams, who withdrew from Stanford with an elbow injury, has more wins (40-1) than Pliskova (38-14) this year and the fourth-seeded Czech player snapped a tie with Williams for most aces on the tour. Pliskova has 361 to Serena's 352.
On Saturday, Pliskova will meet No. 60 Varvara Lepchenko of the United States, who knocked off Mona Barthel of Germany, 6-7 (3), 6-2, 6-3.
After losing a tight first set, Lepchenko rebounded to edge Barthel and reach her second Premier-level semifinal of the season and third of her career. Lepchenko raced out to commanding lead in the second set and eventually put the match away after a 2-2 tie in the third set.
"There have been a lot of surprises this week," Pliskova said. "The seeded players have been struggling and the draw is open for anyone. A lot of girls are playing well and my next match will be tough."
Pliskova will become the eighth Czech woman to be ranked in the top 10 since the current system was introduced in 1975, and three Czech women will be among the top 10 for the first time in WTA history.
Ukraine's Elina Svitolina advanced with a 4-6, 7-5, 6-1 victory over American Alison Riske.
"I was struggling and trying to push myself," Svitolina said. "It was important to react fast and open the angles."
Riske was serving for the match at 5-4 in the second set when the 20th-ranked Svitolina rallied.
"When I was down 4-5 I said to myself 'hit the ball and wait for your chance,' " Svitolina said. "That helped lead me to victory."
Svitolina plays No. 14 Angelique Kerber, who beat No. 7 Agnieszka Radwanska, 4-6, 6-4, 6-4, in the late match.
In doubles play, Spain's second-seeded Anabel Medina Garrigues and Arantxa Parra Santonja topped Chin-Wei Chan (Chinese Taipei) and Darija Jurak (Croatia), 6-3, 7-5.