China's Li Na produced the biggest shock when she beat world number three Svetlana Kuznetsova 7-6, 6-4 in the tennis copmpetition.
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.
Since tennis returned to the Olympic fold in 1988 no man ranked in the world's top five has taken home the singles gold medal but that jinx looks likely to be snapped.
The women's side of the draw appears more open.
Li, the first Chinese woman to reach the top 20 in 2006, put her victory over Kuznetsova down to home court advantage.
"It was her first match too, and she was probably nervous."
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, ditched out 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. He beat Belgium's Steve Darcis 6-4, 7-5.
Americans Serena Williams and Venus Williams 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.