Tokyo - Rio Olympic champion Monica Puig beat seventh seed Petra Kvitova 1-6, 6-4, 6-4 with a dogged display to reach the Pan Pacific Open quarter-finals on Wednesday.
Puig, who beat Germany's Angelique Kerber in last month's Olympic final to capture Puerto Rico's first ever gold medal, recovered from a nightmare start in Tokyo to stun Czech Kvitova, a two-time Wimbledon champion.
"The balls felt heavy today with the roof open," said Puig. "It took some time to adjust to the conditions but I kept fighting and that's what got me through."
In other matches, former world number one Caroline Wozniacki also advanced, the Dane toppling fourth seed Carla Suarez of Spain 7-6, 4-6, 6-4 while Japanese teenager Naomi Osaka upset Slovakia's Dominika Cibulkova 6-2, 6-1.
Osaka, born of a Japanese mother and Haitian father and who models her game on her idol Serena Williams, insisted she paid no attention to rankings after thrashing the world number 12.
"I honestly don't care about that," said the 18-year-old tournament wildcard, herself ranked 66th.
"Maybe she didn't know much about me."
After her upset win, Osaka revealed that her mind had wandered during the match to what she was going to eat for dinner.
Asked to pinpoint how her game has improved over a breakout season, Osaka replied cryptically: "I don't know. To be honest, I kept thinking that I'd fancy some grilled meat."