Left-hander Marketa Vondrousova stunned four-time major champion Naomi Osaka, 6-1, 6-4, in 68 minutes to move into the final eight.
Kei Nishikori avoided the same fate as Osaka by fending off Marcos Giron of the U.S., 7-6 (3), 3-6, 6-1. Ivan Ivashka awaits in the third round.
Belinda Bencic is now 5-0 at the Tokyo Olympics after winning her third-round singles match and second-round doubles contest alongside Viktorija Golubic.
Great Britain’s Liam Broady took down Wimbledon semifinalist Hubert Hurkacz, 7-5, 3-6, 6-3, to spring a second-round upset.
Jessica Pegula and Bethanie Mattek-Sands are a win away from reaching the medal rounds thanks to their 6-1, 6-4 dismissal of Alize Cornet and Fiona Ferro.