Second-seeded Angelique Kerber was bundled out of the WTA New Haven Open on Tuesday, beaten 6-2, 6-4 by unseeded Russian Elena Vesnina.
Vesnina, who had lost all three of her prior encounters with Germany's Kerber, won the first set with ease and recovered from an early break in the second to grab the victory and book her quarter-final berth.
"She's a very tough player, can win on any surface and is a fighter, so it was important for me to get early confidence by winning the first set," Vesnina said. "The second set was up and down with a lot of breaks, but at the key moments I served well."
Vesnina's triumph over the world number 10 was her sixth career victory over a top 10 player and her second this year.
"Going against a top 10 player is always tough because they have a lot of confidence," Vesnina said. "It was important for me to play my game, take my chances, be consistent and be aggressive."
Vesnina next faces Czech Klara Zakopalova, a 6-4, 6-1 winner over Puerto Rico's Monica Puig.
Vesnina is trying to improve on her previous best showing at New Haven, a runner-up finish in 2009.
"I have great memories here from 2009," she said. "I feel really good about this tournament, the atmosphere, the courts. Some players feel certain tournaments are their place. That's how I feel about this tournament."
Kerber said she'd be back on the court Wednesday training for the US Open, the final Grand Slam of the year that starts Monday at Flushing Meadows.
"I tried to fight, but she was playing well and I couldn't find my rhythm," Kerber said. "Sometimes you have days like this."
Defending champion and third seed Petra Kvitova of the Czech Republic launched her campaign Tuesday with a 4-6, 6-3, 6-3 first-round victory over Germany's Annika Beck.
Kvitova was down a set and a break, but converted six of eight break points in the second and third sets to come back.
Former Wimbledon champion Kvitova beat Russian Maria Kirilenko in last year's New Haven final. She next faces US qualifier Alison Riske.
The winner of that match will face Russian Anastasia Pavlyuchenkova, who ousted seventh-seeded German Sabine Lisicki 7-5, 6-3 in a second-round clash.