Serena Williams roared back after falling behind in the third set to beat Ana Ivanovic 2-6, 6-3, 7-5 on Friday night to reach the semifinals of the Bank of the West Classic. The top-seeded and top-ranked Williams dropped three consecutive games to fall behind 4-3 and then ripped seven straight points to right the ship in front of a sellout crowd at Stanford. Andrea Petkovic ended Venus Williams' run in the tournament, beating the former top-ranked player 6-1, 3-6, 7-5 in the quarterfinals. Following a shaky start, the 25th-ranked Venus 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. In other quarterfinals, Varvara Lepchenko took care of qualifier Sachia Vickery 6-1, 6-0 to earn her 400th career win, and third-seeded Angelique Kerber topped Garbine Muguruza, 6-2, 6-1.