American second seed Coco Gauff fell to a shock defeat against Croatia's Donna Vekic in the third round of the women's singles.
Gauff, 20, was among the favourites for a medal at the venue where she reached the French Open final in 2022.
The reigning US Open champion lost 7-6 (9-7) 6-2 against 28-year-old Vekic, who reached the Wimbledon final earlier this month, as she continued her recent renaissance.
German veteran Angelique Kerber - who announced on Friday she will retire after the Games - continued her farewell with a 6-4 6-3 win over Canada's Leylah Fernandez.
American Danielle Collins, another player quitting the sport later this year, is also through to the last 16.
Collins, who spoke in a BBC Sport column earlier this year about why she is retiring despite being in the world's top 10, won 6-0 4-6 6-3 against Colombia's Camila Osario.
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