Serena Williams tops Petkovic in Stanford semisSerena Williams, of the United State, returns the ball to Andrea Petkovic, from Germany, during the first set of a semifinal in the Bank of the West Classic tennis tournament in Stanford, Calif., Saturday, Aug. 2, 2014. (AP Photo/Jeff Chiu)
The top-seeded and top-ranked Williams is 3-0 in finals so far in 2014, winning titles at Brisbane, Miami and Rome.
The tournament is Williams' first since she tried to play through a viral infection that caused her to lose equilibrium during a doubles match with sister Venus at Wimbledon.
Against the ninth-ranked Petkovic, Williams wasted four break points in the final game of the first set before eventually converting her fifth.
In Sunday's final at the hard-court tournament, Williams will face third-seeded Angelique Kerber, a 4-6, 7-6 (4), 6-2 winner over Varvara Lepchenko.
Petkovic was trying to become the ninth player - and first since 2010 - to beat both Venus and Serena Williams in one tournament. Petkovic eliminated Venus on Friday.