Stanford - Teenage qualifier Naomi Osaka of Japan defeated Samantha Stosur 4-6, 7-5 (7), 7-5 in the first round of the Bank of the West Classic on Monday, dealing a blow to the former US Open champion's preparations for this year's Grand Slam at Flushing Meadows.
Osaka, 16, dropped serve for the first time to lose the first set, then rallied for a 5-3 lead in the second before winning three straight points to claim the tiebreaker.
Stosur took a 5-3 lead in the third set, but Osaka went on a roll to win the last four games and finish off the match in two-and-a-half hours.
Osaka next plays either No 8 seeded Andrea Petkovic or Ajla Tomljanovic, who were to meet later Monday.
Karolina Pliskova of the Czech Republic beat Japanese veteran Kimiko Date-Krumm 6-1, 6-3 to set up a second-round match against top-seeded Serena Williams, who had a bye through the first round in her comeback tournament following an illness that ended her run at Wimbledon.
In other first-round matches, American qualifier Sachia Vickery defeated China's Shuai Zhang, 6-1, 6-2 and Varvarel Lepchenko beat Caroline Garcia of France 6-4, 6-3.
"This was a real good confidence booster for me," Vickery said.