American teenager Madison Keys beat No. 8 seed Magalena Rybarikova 6-2, 6-2, in the first round of the $795,000 Bank of the West Classic on Monday.
Sorana Cirstea, the only other seeded player on opening day, beat Ayumi Morita 6-2, 6-2.
Keys, ranked a career-high 44th, played a typical match in topping the 39th-ranked Slovakian for the second time in three career meetings.
Keys recorded four aces and had three double faults. She won 70 percent of her first serve points. In 40 previous matches this season, the 18-year-old phenom averaged 3.8 aces and 1.35 double faults per match while winning 69.7 percent of her first-serve points.
She's still looking for her first WTA Tour title, but she's had her moments in her career. A few months after turning pro on her 14th birthday, she beat Serena Williams 5-1 in a World Team Tennis match. She's 6-5 in Grand Slam events, reaching the third round at both the Australian Open and Wimbledon.
In her last match, Keys took Wimbledon finalist Agnieszka Radwanska to three sets at the All-England Club.
Cirstea, seeded fifth, played a clean game, with few unforced errors and no double faults, in beating the 51st-ranked Japanese player.
Cirstea, ranked 32nd, played Morita for the third time this season and sixth overall.
In other first-round matches, Belarus' Govortsova downed Germany's Julia Goerges, 7-6 (2), 6-2; Italian Francesca Schiavone topped American and former Stanford All-American Mallory Burdette 7-5, 6-3; and Sweden's Stefanie Voegele bettered wild card Ajla Tomljanovic of Croatia 6-4, 6-0.