World number 18 Madison Keys won her return to the WTA Tour, after almost a one-month hiatus following Wimbledon.
Madison Keys won her first match since appearing in the Wimbledon quarter-finals, cruising past Aleksandra Krunic at the Bank of the West Classic on Monday.
Keys, seeded seventh, accounted for Krunic 6-3 6-0, overcoming a tight opening before racing to the victory in just under 53 minutes in Stanford.
The American, who lost in the last eight at SW19 to Agnieszka Radwanska, won nine games in a row and 15 of the last 16 points to career into the second round at the WTA Premier hard-court event.
Sixth seed Andrea Petkovic avoided an upset, seeing off fellow German Carina Witthoeft 5-7 6-1 6-3.
Petkovic will take on another compatriot in the second round, after Mona Barthel got past Canadian wildcard Carol Zhao 6-3 6-0.
Zhao provided plenty of first-set resilience, saving as many as eight break points - including five to hold in the first game - but ultimately she was broken thrice as Barthel took control.
Barthel went on to sweep the second set in 23 minutes.
Qualifiers Misaki Doi and Nicole Gibbs made the most of their main-draw berths with first-round wins.
Japan's Doi proved to be too good for Catherine Bellis, a wildcard, 6-3 7-6 (7-3), while Gibbs defeated Caroline Garcia for the second time in 12 months - adding to her US Open victory last year with a 6-4 7-5 triumph.
Doi earned a date in the last 16 with second seed Radwanska - the top-four seeds receiving byes past the first round.
Alison Riske made it three Americans through to the last 16 from a possible four in Monday's action, edging out Tatjana Maria 6-2 5-7 6-1.
Riske next meets third seed and world number 10 Carla Suarez Navarro.