China's Li Na produced the biggest shock so far of the Olympics tennis tournament to beat world number three Svetlana Kuznetsova 7-6 6-4.
Li sparked joyful scenes at the end of a day that had seen men's heavyweights Roger Federer, Rafael Nadal and Noval Djokovic flex their muscles with first round victories.
Li, who reached the semi-finals at Wimbledon last month was the first Chinese woman to reach the top 20 in 2006.
Federer, who will lose his world number one spot next week to Nadal, eased past Russian Dmitry Tursunov 6-4 6-2, while Serbian Djokovic disposed of American Robby Ginepri 6-4 6-4.
Only Spain's Nadal was troubled. He kicked off a manic day on centre court in front of Spain's Prince Felipe with a 6-2 3-6 6-2 victory over Italian Potito Starace.
But three other strong contenders for the men's singles gold, Spaniards David Ferrer and Nicolas Almagro and Britain's Andy Murray, lost in the first round.
Sixth seed Murray lost 7-6 6-4 to Taiwan's Lu Yen-Hsun while number five Ferrer was bundled out by Serbian Janko Tipsarevic 7-6 6-2. Almagro lost to Frenchman Gael Monfils.
Nicolas Massu, the surprise gold medallist in singles and doubles four years ago in Athens, survived, beating Belgium's Steve Darcis 6-4 7-5.
Americans Serena Williams and Venus Williams, most people's favourites to contest the women's gold medal match on Saturday, had easy victories.
Serena completed a 6-3 6-1 victory over Olga Govortsova of Belarus while Venus, playing her first singles match since beating Serena in the Wimbledon final last month, beat Swiss Timea Bacsinzky 6-3 6-2.
Serbia's Jelena Jankovic also made it through with a 6-3 6-3 victory over Zimbabwe's Cara Black.