Petkovic beats Venus Williams in Bank of the West Classic quarterfinals

Andrea Petkovic, from Germany, celebrates after defeating Venus Williams, of the United States, at the Bank of the West Classic tennis tournament in Stanford, Calif., Friday, Aug. 1, 2014. Petkovic won 6-1, 3-6, 7-5. (AP Photo/Jeff Chiu) (The Associated Press)

Andrea Petkovic, from Germany, returns the ball to Venus Williams, of the United States, during the second set of their match in the Bank of the West Classic tennis tournament in Stanford, Calif., Friday, Aug. 1, 2014. Petkovic won 6-1, 3-6, 7-5. (AP Photo/Jeff Chiu) (The Associated Press)

Eighth-seeded Andrea Petkovic ended Venus Williams' run in the Bank of the West Classic, beating the former top-ranked player 6-1, 3-6, 7-5 in the quarterfinals Friday.

Following a shaky start, the 25th-ranked Williams picked up her game in the second set and appeared poised to take over the match. The 18th-ranked Petkovic rallied in the third set, taking leads of 3-1 and 5-3, to beat Williams in their first meeting since Luxembourg in 2012.

Petkovic, making her debut in the tournament, is looking for her third title of the season. She also reached the semifinal at the French Open.

Varvara Lepchenko took care of qualifier Sachia Vickery, 6-1, 6-0, to earn her 400th career win, which includes all levels. She will meet third-seeded Angelique Kerber on Saturday night. Kerber topped Garbine Muguruza 6-2, 6-1.