Stanford: Danish top seed Caroline Wozniacki slumped to a shock 6-4, 6-2 defeat to 60th-ranked Varvara Lepchenko in her opening match of the WTA Tour`s Stanford tournament on Thursday.
World number five Wozniacki was kicking off her US hardcourt campaign in her first match since playing Wimbledon last month.
After dropping the first set against Lepchenko, Wozniacki lost her serve twice early in the second and fell behind 4-1 before losing the set 6-2.
"I just followed my plan," said Lepchenko, who hammered 26 winners and had just 17 unforced errors. "The plan was to be aggressive, and when I had to be on defense, be on defense.
"But when I had my chances I was aggressive and I did that until the last point, and it worked out for me tonight."
The Uzbek-born American needed only 92 minutes to post her third career win over a top-five opponent and book a quarter-final date with Germany`s 63rd-ranked Mona Barthel.
Wozniacki had received a bye through the first round.
Polish second seed Agnieszka Radwanska outlasted Japan`s Misaki Doi 1-6, 6-2, 6-0 to reach a last-eight matchup against German fifth seed Angelique Kerber, who downed Croatian Ana Konjuh 6-4, 6-3.
Czech fourth seed Karolina Pliskova, who advanced 7-5, 6-2 over Japan`s Kimiko Date-Krumm, will meet Croatian Ajla Tomljanovic in the quarter-finals.
The other quarter-final will be between American Alison Riske and Ukraine`s Elina Svitolina.
The event lost its star headliner when world number one Serena Williams withdrew. The American is going for her fifth consecutive Grand Slam singles title and the 22nd of her career at the US Open next month in New York.