Maria Sharapova reached her third quarter-final of the year at the Tianjin Open with a straight-sets victory over Magda Linette on Thursday.
Thursday's victory marks the third last-eight showing of the season for the five-time grand-slam victor from Russian Federation, fighting to revive her ranking after serving a 15-month suspension for using an illegal doctor-prescribed medication.
Petra Kvitova, also seeded second, suffered a shock defeat in her first-round match at the Tianjin Open.
Second seed Williams, playing in her second match since the US Open, lost 7-5 6-2 to the Japanese, who is ranked 64th in the world.
The Russian's first-serve percentage was only 59 per cent, but she was able to overpower the Pole and saved nine break points to get the job done in two sets, setting up a meeting with Stefanie Voegele - who ousted Yulia Putintseva in the final match of the day.
Third seed and defending champion Peng Shuai eased into the last eight with a 6-2 6-2 win over Wang Xiyu while Christina McHale upset sixth seed Maria Sakkari 6-0 6-4.
Italian qualifier Sara Errani continued her surge on her comeback from a two-month doping ban with a 6-4, 4-6, 6-3 victory over Brazil's Beatriz Haddad Maia.
Peng Shuai, a victor of singles and doubles titles at this tournament a year ago, cruised to a 6-2 6-2 win over Wang Xiyu in an all-Chinese second-round encounter.