STANFORD, Calif. (AP) American Varvara Lepchenko beat top-seeded Caroline Wozniacki 6-4, 6-2 in the second round of the Bank of the West Classic on Thursday night.
''It was kind of surprising,'' Lepchenko said. ''The first match helped coming into this match. I felt the ball wasn't coming at me as fast. My first match, it really seemed to be coming fast.''
The fifth-ranked Wozniacki, the Danish player who reached the round of 16 at Wimbledon, became the fourth-seeded player to drop out and the second No. 1 seed. Top-ranked Serena Williams pulled out with an elbow injury on the eve of the draw.
''Physically I wasn't 100 percent,'' Wozniacki said. ''I still thought I had enough to play and beat her. The ball was coming at me so fast I couldn't get into a rhythm. It wasn't pretty out there.''
The 60th-ranked Lepchenko reached the quarterfinals after failing to get out of the first round in her last seven events. She beat a top-20 opponent for the first time this year and knocked off a top-five opponent for the third time in her career.
''I took some time to get back into shape,'' Lepchenko said. ''Right now I feel like I am in better condition than I was earlier in the year.''
The players endured four brief rain delays in the second set.
''I felt the sky didn't want me to win,'' Lepchenko said. ''Every time I would get some momentum, start going for winners, it would start raining. It was like a roller coaster. It' OK to rain but not every other game.''
Wozniacki, who owns the longest streak, at six years, of finishing the season among the top 10, said the rain was bothersome but not a factor.
''You warm up and then get cold,'' she said. ''You'd rather play straight through, but she just played well and beat me.''
Lepchenko will face Germany's Mona Barthel on Friday in the quarterfinals at The Taube Family Tennis Center. Barthel advanced Wednesday with a victory over sixth-seeded Andrea Petkovic.
Earlier, second-seeded Agnieszka Radwanska of Poland beat Japanese qualifier Misaki Doi 1-6, 6-2, 6-0 to set up a match against fifth-seeded Angelique Kerber of Germany. Kerber, a finalist at Stanford last year, beat Ana Konjuh of Croatia 6-3, 6-4.
The seventh-ranked Radwanska, playing for the first time since the Wimbledon semifinals, was down 0-2 in the second set before rattling off 12 consecutive games to set up a meeting with fifth-seeded Angelique Kerber in Friday's quarterfinals.
''It was just a weird match with such a slow start and then winning 12 games in a row,'' Radwanska said. ''The ball was flying a lot and I really didn't practice much the last few days. For me, it was a new start, new courts, new balls and a new tournament.''
Fourth-seeded Karolina Pliskova of the Czech Republic topped Japanese qualifier Kimiko Date-Krumm, at 44 the oldest player on the tour, 7-5, 6-2.
''She plays so cleverly,'' Pliskova said. ''I get the feeling she could play the next five years. I was expecting a tough match and the first set was really tough.''
Pliskova will face Croatia's Ajla Tomljanovic.
In the other quarterfinal, American Alison Riske will play eighth-seeded Elina Svitolina of Ukraine.