Venus Williams was knocked out in the second round of the Hong Kong Open by 19-year-old Naomi Osaka.
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.
Petra Kvitova, also seeded second, suffered a shock defeat in her first-round match at the Tianjin Open.
The Czech coughed up eight double faults as she lost 6-4 5-7 6-4 to world number 113 Zhu Lin of China.
Kvitova has now lost five of the seven three-set matches she has played this year.
Source: BBC Sport