Full coverage: 2016 Rio Olympics

China's Zheng Saisai faced a difficult first round match in the form of the World Number Four from Poland, Agnieszka Radwanska. But Zheng came in with a glimmer of hope due to Radwanska's rather poor play of late, and that glimmer would quickly grow as the match progressed.

Chinese player reaches Second Round in women's tennis 6-4, 7-5

Zheng looks good from the outset, and after fending off two break points in the eighth game, the Chinese would claim the first set 6-4. The second frame would be similar, and Zheng would earn a break to take a 3-1 lead. But Radwanska would get back on serve, only to be broken again late, as Zheng takes the set 7-5 to complete the huge upset. Next up for the Chinese is Daria Kasatkina of Russia.

Belgian eliminates Sydney 2000 gold medalist 4-6, 6-3, 7-6

Radwanska would not be the only top star eliminated early, as American fifth seed Venus Williams squanders a 4-1 lead in the deciding set of her encounter with fellow veteran Kirsten Flipkens. Williams won the first set 6-4, but her determined opponent comes back to claim the second 6-3. The decider would go to a tiebreak, with the Belgian holding her nerve in a tense affair, to come away with a memorable win in over three hours.

Another victor was China's Zhang Shuai, who upset Timea Bacsinszky of Switzerland to reach the Second Round. But Zhang's compatriot Peng Shuai was not so fortunate, losing a close three setter to Heather Watson of Great Britain. Carla Suarez Navarro had a good start by beating Ana Ivanovic of Serbia.

On the men's side, Japanese fourth seed Kei Nishikori cruised through in straight sets against Spain's Albert Ramos-Vinolas. French fifth seed Jo-Wilfried Tsonga also got the job done in three against Tunisia's Malek Jaziri. And Marin Cilic of Croatia wiped the floor with Bulgaria's Grigor Dimitrov.