American swimmer Michael Phelps won the men's 100m butterfly on Friday for his 17th Olympic gold and 21st medal overall, as the United States overtook China on the top of the medals table thanks to a gold rush in the pool.

Phelps clocked 51.21 seconds for the 100m title, adding to his victories earlier in the 200m IM and 4x200m freestyle relay.

Chad le Clos of South Africa and Evgeny Korotyshkin of Russia touched in 51.44 to share the silver medal.

Until Thursday no man had won an Olympic swimming title in three separate Games, Phelps ended that record with his win in the 200-meter medley and Friday's win means he has repeated the same feat in this race.

"I am just happy that the last one was a win. That is all I really wanted and this was by a bigger winning margin than the last two combined," said the 27-year-old Phelps, who has announced to retire after the London Games.

On Sunday, Phelps will take part in his last race - the 4x100m medley relay.

Fellow American Missy Franklin also claimed her third gold medal of the games with a world record-breaking swim in the 200m backstroke. The 17-year-old, who had already won the 100m backstroke and formed part of the U.S. women's 4x100m freestyle relay, touched home in 2:04.06, eclipsing the previous best set by Kirsty Coventry in Rome in 2009.

Coventry was in the race too, but was unable to make the podium.

"I knew I was just going to take it out and have fun and that's what I did. I am the happiest girl alive," said Franklin.

With the women's 4x100m medley to be contested on Saturday, Franklin has a chance to take her fourth gold, which will make her the most successful swimmer in London.

Fifteen-year-old Katie Ledecky, the youngest member of the U.S. team, upset local favorite and defending champion Rebecca Adlington in the 800m freestyle in 8:14.63.

In the last event of the night, Frenchman Florent Manaudou denied a U.S. clean sweep by taking the men's 50m freestyle in 21.34 seconds, edging American Cullen Jones into second in 21.54 and Brazilian world record holder Cesar Cielo third in 21.59.

The United States now leads the medals count with 43, including 21 gold. China is in second place with 20 gold, 13 silver and nine bronze after claiming two gold on Day 7.

Zhang Nan and Zhao Yunlei won the mixed doubles gold at the badminton tournament, beating Xu Chen and Ma Jin 21-11 21-17 in the all-Chinese final.

Also, China is assured of the women's singles title, with top seed Wang Yihan and third seed Li Xuerui winning their respective semifinals.

In men's singles, Malaysia's Lee Chong Wei set up an anticipated championship rematch with defending champion Lin Dan. Lee beat China's Chen Long 21-13, 21-14, and Lin overcame South Korean Lee Hyun-il 21-12, 21-10.

"I'm really happy that I'm going to meet my old opponent," said 28-year-old Lin. "We are the same age and we get to play at the final of the Olympics. It's really good as it shows that we have not been knocked out by time."

Chinese world number one Dong Dong broke down in tears as he snatched the Olympic men's trampoline title.

Guo Shuang gave China second silver medal in cycling events when she was narrowly beaten by Briton Victoria Pendleton in the women's keirin, a day after she and teammate Gong Jinjie won the team sprint event only to be relegated soon after an "illegal changeover".

Germany's Miriam Welte and Kristina Vogel were crowned inaugural Olympic women's team sprint champions following the relegation of China.

But the Chinese cycling team appealed the ruling on Friday. In an official statement, the Chinese team said: "Gong Jinjie and her teammate Guo Shuang beat the record twice, in the qualifying and the first round. In the final, Chinese team beat Germany with a result of 32.617, but was relegated to second place by the referee who did not show any video replay."

"Our gold was robbed," China's French coach Daniel Morelon said on Friday.

"Even if there is no appeal process in cycling, we still wanted the world to know that this is unfair and shameful for the Olympic spirit," he said.

"After the race, I came to the referee, who did not show me any video replay," said Morelon, a cycling legend who won four Olympic golds in his career.

"I watched the race video again and again, after I returned to the Athletes' Village. I looked over the rules and regulations of the sport and concluded the Chinese didn't violate any rules."

In tennis, Maria Sharapova breezed into her first Olympic final as the 25-year-old Russian defeated compatriot Maria Kirilenko 6-2, 6-3 in the semifinals.

Sharapova could complete the "Super Slam" of titles in all four majors and the Olympic singles if she beat Wimbledon champion Serena Williams in Saturday's final. Williams brushed aside world number one Victoria Azarenka 6-1, 6-2 in the other semifinal.

Seven-time Wimbledon champion Roger Federer is just one match away from his first Olympic singles title after edging Juan Martin Potro of Argentina 3-6, 7-6 (7-5), 19-17 in four hours and 26 minutes, the longest three-set men's singles match in the Open era.

On the first day of athletics action, Poland's Tomasz Majewski clinched the men's shot put gold with a throw of 21.89 meters. Germany's David Storl claimed silver with 21.86m and American Reese Hoffa bronze with 21.23.

Ethiopia's Tirunesh Dibaba became the first woman to win the Olympic 10,000m final for the second straight time. She crossed the line in 30 minutes 20.75 seconds, a world best time for 2012.