Sixth-seeded Andrea Petkovic beat fellow German Carina Witthoeft in a first round match of the $731,000 Bank of the West Classic on Monday.

Petkovic won 5-7, 6-1, 6-3.

American teenager CiCi Bellis lost her match to qualifier Misaki Doi, 6-3, 7-6 (3). Bellis was the youngest player in 15 years to win a match at the U.S. Open last year.

Qualifier Nicole Gibbs, who won a pair of NCAA singles titles while at Stanford, beat France's Caroline Garcia 6-4, 7-5, for her first WTA main draw victory since April.

American Alison Riske topped German Tatjana Maria 6-2, 5-7, 6-1 to earn a date with third-seeded Carla Suarez Navarro in the second round. Doi meets No. 2-seed Agnieszka Radwanska on Thursday.

World No. 1 and defending champion Serena Williams withdrew last week due to injury.