Reigning Wimbledon champion Elena Rybakina of Kazakhstan lost to seventh seed Daria Kasatkina on Monday in a first-round match at the WTA San Jose hardcourt tournament.
World number 12 Kasatkina won 11 of the last 12 games to win 1-6, 6-2, 6-0 in Rybakina's debut match since winning her first major title.
Kasatkina will next play American Taylor Townsend, who eliminated Australian Storm Sanders 6-1, 6-4 at the US Open hardcourt tuneup event.
After 23rd-ranked Rybakina rolled through the opening set in 35 minutes, Kasatkina dominated the final two.
American Madison Keys, the 2017 US Open runner-up, defeated China's Zhang Shuai 6-4, 6-2 to book a second-round date against Tunisian Ons Jabeur, who lost to Rybakina in last month's Wimbledon final.
Japan's Naomi Osaka, a four-time Grand Slam champion, and American sixth seed Coco Gauff would meet in a second-round matchup if both win their openers on Tuesday.
Former world number one Osaka will face China's Zheng Qinwen while 18-year-old Gauff, this year's French Open runner-up, meets Anhelina Kalinina of Ukraine.
Osaka, whose ranking has slid to 41st, has not played since losing her first-round match at the French Open. She had suffered a right ankle injury in a tune-up event at Madrid.
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