China's Zheng Saisai clinched her doubles victory on Sunday following her first round surprise win over fourth seed Agnieszka Radwanska of Poland in the singles of the Rio Olympic Games.

Zheng and Xu Yifan eased past Ukraine's twin sisters Liudmyla and Nadiia Kichenok 6-0, 6-3 in 84 minutes. Zheng, who was the last to qualify for the women's singles event on Tuesday following several drop-outs in the main draw, eliminated the world number five Radwanska 6-4, 7-5 in one hour and 39 minutes on Saturday.

In an earlier singles match, Wang Qiang of China lost to Russian 8th seed Svetlana Kuznetsova 6-1, 4-6, 6-0 in the first round.

Top seed and multi-Grand Slam winner Serena Williams opened her title defense with an easy first round 6-4, 6-2 win over Australian Daria Gavrilova on Sunday.

"It feels good to win my first match. No one wants to go out in the first round. It's always a little tricky and you always want to at least win a match, so I feel really good about that. It wasn't that easy. She has a lot of big wins and she plays really well," she said.

Serena's older sister Venus, however, went out in the first round on Saturday, losing to Belgium's Kirsten Flipkens 6-4, 3-6, 7-5.

Second seed Angelique Kerber of Germany progressed, beating Colombian Mariana Duque-Marino 6-3, 7-5 to book a second round meeting with Canada's Eugenie Bouchard.

In the men's competition, defending champion Andy Murray beat Serbian Viktor Troicki 6-3, 6-2 while third seed Rafael Nadal also booked an easy win, beating Federico Delbonis of Argentina 6-2, 6-1 in the first round.