Rio de Janeiro, Aug 7 (IANS): China's Zheng Saisai pulled off a major early upset in Rio Olympics women's tennis event, eliminating fourth-seeded Pole Agnieszka Radwanska 6-4, 7-5 in first-round action.

Zheng, a 22-year-old who has never won a WTA Tour title and is competing in her first Olympics, was considered an underdog against the 2012 Wimbledon runner-up who has won 18 career titles, reports Efe.

But the 64th-ranked Zheng held her nerve down the stretch to move into the second round on Saturday where she will face 19-year-old Russian Daria Kasatkina.

Another Chinese woman, 51st-ranked Zhang Shuai, pulled off a mild upset by edging Switzerland's Timea Bacsinszky 6-7 (4-7), 6-4, 7-6 (9-7). But her compatriot Peng Shuai lost to Britain's Heather Watson 6-4, 6-7 (5-7), 6-3.

In men's singles, fourth-seeded Japanese Kei Nishikori cruised past Spaniard Albert Ramos-Vinolas 6-2, 6-4, and his compatriot, 118th-ranked Taro Daniel, stunned American No. 14 seed Jack Sock 6-4, 6-4.

Ninth-seeded Croatian Marin Cilic, the 2014 US Open champion, comfortably navigated a difficult first-round clash against Bulgaria's Grigor Dimitrov, winning 6-1, 6-4.