STANFORD, CALIF. — Varvara Lepchenko had never had much luck against Caroline Wozniacki. Until Thursday at the Bank of the West Classic, she was winless in four career matches against the Dane, including a 6-0, 6-1 drubbing in their most recent meeting last year in Miami.

But the Uzbeki-turned-American, a tour veteran at age 29, isn’t necessarily the same player Wozniacki had faced in the past. Granted, the world No. 5 was playing only her first match of the Emirates Airline US Open Series, and she has been nursing both a bad back and a sore leg, but Wozniacki’s woes shouldn’t detract from the fact that she was simply outhit by a more confident, more aggressive version of Lepchenko than she had ever encountered before.

One of 11 American women in the top 100 (she was granted political asylum and changed her nationality in 2007), Lepchenko smacked 26 winners to just 17 unforced errors in dismantling the tournament’s top seed 6-4, 6-2 for only the fifth top-10 win of her career. It was a much-needed result in many respects. After a fairly solid start to the year, including a Brisbane semifinal, Lepchenko struggled through the clay- and grass-court campaigns. She arrived in California having fallen in the first round of her last seven events. Ouch.

“The beginning of my year was stopped by being very sick,” said the lefthander, who is still seeking her first career WTA title. “It was so hard for me to restart everything.  Things started popping up — injuries. I had to stop and start. I never had a full recovery. But I kept on going. Finally, after Wimbledon, I said, ‘That’s it. I really have to take time now.’”

Not that she paused for long. The No. 60-ranked Pennsylvanian said she began working on her physical conditioning the very day after she fell to eventual runner-up Garbine Muguruza in the first round at the All England Club, but she limited her on-court time to just three days that first week. After that, it was back to business as usual — two fitness sessions and two hitting sessions every day.

“I worked pretty hard to get myself to this point,” said Lepchenko, who as she heads into the quarterfinals says she’s in the best shape she’s been in all year.

Lepchenko has never done much damage at the majors. Her best result was a round of 16 showing at Roland Garros in 2012, the same year she finished with a career-high ranking of No. 21. So it would be a stretch to think she’ll suddenly be a second-week threat when the US Open gets underway later this month. But wins over elite  players like Wozniacki will only boost her confidence as she heads into the year-end Grand Slam. If she can continue to do what she did to one of the best counterpunchers in the business — be aggressive with her shots, show variety with her serve, keep opponents from settling into their comfort zones — she’ll only further show the competition that she’s a dangerous draw on any surface.  

“I hear some people say that I’m not an aggressive player and I don’t like fast courts, but these courts [Stanford] are pretty fast and I do pretty well, so it gives me a lot of confidence that I can play on every court,” she said.

Lepchenko will next face Mona Barthel in Friday’s quarterfinal. She’s 1-1 against the German, whom she defeated last year at the US Open 6-4, 6-0.