Ye Shiwen and Sun Yang seized gold as China issued a challenge to U.S. supremacy in London's Olympic pool on July 28, amid a day of disappointment for U.S. swimming star Michael Phelps.
Ye won the women's 400-meter individual medley at the Olympic Park's Aquatics Center, setting a world record.
The 16-year-old won gold in 4 minutes, 28.43 seconds -- more than a second faster than defending champion Stephanie Rice's time from the 2008 Beijing Olympics.
Elizabeth Beisel of the United States took silver ahead of China's Li Xuanxu.
Earlier in the day, Sun became the first Chinese male swimmer to win Olympic swimming gold when he won the 400-meter freestyle.
Sun, 20, won in three minutes 40.14 seconds, missing out on German Paul Biedermann's 2009 world record by just 0.07 seconds.
South Korea's defending champion Park Tae-Hwan took silver while Peter Vanderkaay of the United States finished third.
However, it was a disappointing night for Phelps who finished with no medals in fourth place in the 400-meter individual medley swimming race.
He was outclassed by his U.S. teammate and arch rival Ryan Lochte.
Brazil's Thiago Pereira took silver while Japan's Kosuke Hagino finished third.
Speculation was running high over whether Phelps -– who already has 16 Olympic medals, 14 of them gold, from earlier Games -– would win the event.
He hopes in London to win enough races to become the most prolific Olympic medalist of all time. The greatest-ever record of 18 medals is held by Soviet gymnast Larisa Latynina.
After the lavish opening ceremony on July 27, it was a packed first day of full action during which China gathered a total of six Olympic medals, four of them gold. Both Italy and the United States received five medals. Kazakhstan, Russia, Romania, and Serbia won one medal each.
Alexandr Vinokourov of Kazakhstan won a gold medal in the men's road race, outsprinting Colombian Rigoberto Uran, who took silver. Alexander Kristoff of Norway won the bronze medal. Vinokourov was the silver medalist in Sydney 12 years ago.
Arsen Galstyan of Russia won the men's judo 60-kilogram gold medal, becoming Russia's first Olympic judo winner since the collapse of the former Soviet Union in 1991.
Galstyan, 23, beat Japan's Hiroaki Hiraoka with an ippon, an automatic winning score, just 40 seconds into their final clash. He dedicated the gold medal to the people of his hometown of Krasnodar, where 170 people died in devastating flooding earlier this month.
Uzbekistan's Rishod Sobirov secured bronze by beating France's Sofiane Milous. Brazil's Kitadai Felipe took the other bronze, beating Italy's Elio Verde.
In women's 60-kilogram judo, Brazil's Sarah Menezes won gold by beating defending Olympic champion Alina Dumitru of Romania, who received silver. The bronze medals went to Hungarian Eva Csernoviczki and Charline van Snick of Belgium.
Andrija Zlatic of Serbia took bronze in the men's 10-meter air pistol event. The competition was won by South Korean Jin Jong-oh, who took gold ahead of Italy's Luca Tesconi.
July 28 also witnessed the first failed drugs test of the Games after Albanian weightlifter Hysen Pulaku tested positive for the banned steroid stanozolol in London on July 23. About a dozen athletes have been barred in the lead-up to the Games for pre-competition doping offenses.
Meanwhile, organizers were investigating why several venues had swathes of empty seats despite overwhelming pre-tournament demand for tickets.
With reporting by AFP and dpa