Venus Williams and Agnieszka Radwanska suffered surprise exits from the Hong Kong Open, while Barbora Strycova battled past Madison Brengle in Linz.
Second seed Williams coasted past Risa Ozaki in the first round but came unstuck against another Japanese on Wednesday, losing 7-5 6-2 to Naomi Osaka.
Osaka secured a crucial break with the 37-year-old looking to serve out the first set and she maintained that momentum to secure the biggest win of her career in terms of rankings.
Teenager Osaka cited Williams as one of her idols after losing to the veteran at Wimbledon this year, and she was elated to have come out on top this time around.
"Today I accomplished one of my dreams," Osaka posted on Twitter after confirming a quarter-final against Anastasia Pavlyuchenkova, who beat Jacqueline Cako 7-6 (7-3) 6-1.
Another seed to tumble out was Radwanska, who let a one-set lead slip to Sam Stosur as she suffered a 3-6 6-4 6-0 loss. However, Daria Gavrilova avoided the same fate with a 6-1 2-6 6-2 triumph over Shelby Rogers.
At the Linz Open, second seed Strycova staved off nine break points as she dug in for a 7-5 6-3 win against Brengle, while Sorana Cirstea also won in straight sets.
Carina Witthoeft set up a second-round clash with top seed Magdalena Rybarikova by downing wildcard Barbara Haas 6-3 4-6 6-4.
Belinda Bencic fought past Lara Arruabarrena to reach the last eight, where she will face qualifier Mihaela Buzarnescu following the Romanian's 4-6 7-6 (7-1) 6-1 win over Ajla Tomljanovic.
