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.
Also in Tianjin, Russia's Maria Sharapova registered a straight-set win over Romanian Irina-Camelia Begu to reach the second round.
The former world number one, now ranked 86th in the world, took just 82 minutes to beat Begu 6-4 6-2.
Sharapova converted all four of her break points to set up a meeting with Poland's Magda Linette.
After rain interrupted the opening days of the tournament, Sharapova said: "It's been a long wait for everyone, it hasn't been easy.
"I had a first practice outdoors, and then I think it rained for 70 hours straight, unfortunately for the players.
"The good thing about this tournament is it has indoors, it's pretty state-of-the-art, and I trained at the National Tennis Centre as well, which was very impressive."