Stanford: Unseeded Varvara Lepchenko toppled second-seeded Agnieszka Radwanska 6-3, 3-6, 6-4 on Wednesday to reach the quarter-finals of the WTA hardcourt tournament at Stanford.
Poland`s Radwanska, ranked fifth in the world, had never lost to the 59th-ranked American left-hander in five prior meetings.
But in her first match since a fourth-round exit at Wimbledon, Radwanska couldn`t hold on to a 3-1 lead in the opening set.
She broke Lepchenko three times to take the second set, but was on the defensive after an early break in the third.
Radwanska broke back to knot the set at 3-3, but Lepchenko broke again for a 5-4 lead and seized her chance for a first career victory over a top-five player, holding serve in the final game to seal the triumph after two hours and 35 minutes.
With the US Open due to start on August 25, it was another disappointing outing for Radwanska, whose fourth-round straight-set loss to Ekaterina Makarova at Wimbledon followed a third-round exit at Roland Garros.
Radwanska has yet to lift a trophy in 2014, and hasn`t reached a final since a runner-up finish to Italian Flavia Pennetta at Indian Wells in March. Lepchenko next faces US qualifier Sachia Vickery, a 6-7 (4/7), 6-2, t-1 winner over Puerto Rico`s Monica Puig.
World number one and top seed Serena Williams was also trying to get her US Open build-up off and running on Wednesday, taking on Czech Karolina Pliskova.
The match marked Williams` return to action for the first time since her disappointing Wimbledon campaign, in which her abrupt third round exit in singles was followed by a disturbing doubles departure.
Williams looked weak and woozy in a doubles match with her sister Venus, in which the Americans eventually retired while trailing.
Serena said this week she had taken three weeks off and was fully recovered from the virus that was the culprit.