Phelps rallied to win the 100-meter butterfly on Friday for his third gold of the London Games and No. 17 of his career. The American was next-to-last at the turn but closed strong to finish in 51.21 seconds, just ahead of Chad le Clos of South Africa and Evgeny Korotyshkin of Russia.
"I'm just happy that the last one was a win," Phelps said. "That's all I really wanted coming into the night."
It was Phelps' third consecutive win in the event at the Olympics. He has said he will retire after the games, so his final Olympic race will be the 4x100 medley relay Saturday.
Phelps' 21st medal was part of another big night at the pool for the U.S., led by a pair of teenagers.
Missy Franklin set a world record in the 200 backstroke for the 17-year-old's third gold in London. And right after Phelps was done, 15-year-old Katie Ledecky - the youngest member of the U.S. team - nearly broke the world record while winning the 800 freestyle, denying Britain's Rebecca Adlington a repeat before her home fans.
Florent Manaudou of France took the 50 freestyle in 21.34 seconds. American Cullen Jones grabbed the silver medal and Brazil's Cesar Cielo was third.
Phelps' race was only slightly faster than a Saudi woman's appearance in the judo tournament, but it was still being hailed as a victory for women in the Gulf kingdom.
Wojdan Ali Seraj Abdulrahim Shahrkhani became the first Saudi woman to compete at the Olympics when she lost her judo fight in 82 seconds. And she only made it to the mat after a compromise between Olympic organizers, the international judo federation and Saudi officials cleared the way for her to wear a modified hijab.
The crowd roared right before Shahrkhani's fight against Puerto Rico's Melissa Mojica. The Saudi, wearing judo dress and what appeared to be a tight-fitting black cap, looked tentative and cautious on her feet, and Mojica eventually grabbed Shahrkhani and flipped her onto her back, ending the match.
As she rose to her feet, Shahrkhani gently reached for her head to make sure the hijab was still in place. It was, and the two women bowed to each other and left to a loud ovation.
Afterward, the teenager walked with her father past journalists and TV cameras.
"I am happy to be at the Olympics," she whispered in Arabic, her brother, Hassan, holding both her arms. "Unfortunately, we did not win a medal, but in the future we will and I will be a star for women's participation."
Olympic Stadium was packed for the first time since the opening ceremony, and heptathlete Jessica Ennis gave the delirious crowd exactly what it was hoping to see.
Ennis finished the 100-meter hurdles in 12.54 seconds, the fastest time ever in the heptathlon's first event and one of the highlights on a raucous opening day for track and field.
Ennis' time matched Dawn Harper's gold-winning burst in the 100-meter hurdle final at the Beijing Games - and would've been good enough to take that title at the 1992, 1996 and 2000 Olympics.
"Amazing. So loud. When you step up to jump or get in your blocks, they really get behind you. It's a great feeling," Ennis said of the home crowd.
Poland's Tomasz Majewski (men's shot put) and Ethiopia's Tirunesh Dibaba (women's 10,000 meters) won the first gold medals in track and field, and world champion Carmelita Jeter of the U.S. led the 100-meter heats with a time of 10.83 seconds.
There were more stirring moments as Wimbledon, where Roger Federer was pushed to the limit in his pursuit of his first Olympic singles medal.
Like so many other times on Centre Court, he delivered.
Federer rallied past Juan Martin del Potro of Argentina 3-6, 7-6 (5), 19-17 to reach the final. At 4 hours, 26 minutes, it was the longest three-set men's match of the Open era.
"I was very tense at certain times," Federer said. "I was seeing myself as a loser many times during the match."
Federer faces another tough challenge when he meets Britain's Andy Murray in Sunday's gold-medal match. Murray, who advanced with a 7-5, 7-5 victory over Novak Djokovic, lost to Federer in the Wimbledon final last month.
Serena Williams also clinched her first Olympic singles medal, beating No. 1-seeded Victoria Azarenka 6-1, 6-2. On Saturday, Williams will face first-time Olympian Maria Sharapova, who beat Russian teammate Maria Kirilenko 6-2, 6-3.
U.S. soccer star Abby Wambach is battling Achilles tendinitis while dealing with a constant stream of defenders determined to shut her down in the London Olympics.
She just keeps scoring anyway.
The 32-year-old striker slid onto a pass in the 27th minute to knock home her fourth goal of the tournament and then celebrated with a cartwheel in the United States' 2-0 win over New Zealand in the quarterfinals of the Olympic tournament.
Sydney Leroux added an insurance goal in the 87th minute for the two-time defending Olympic champion Americans, who will play Canada in Manchester on Monday. Canada advanced with a 2-0 victory over host Britain.
France faces Japan in the other semifinal. Yuki Ogimi scored and set up another goal to give world champion Japan a 2-0 victory over Brazil, and France reached the next round by edging Sweden 2-1 on goals by defenders Laura Georges and Wendie Renard.
The rest of the Olympic action Friday:
The U.S. men's boxing team is done in London.
All nine were eliminated after losses by flyweight Rau'shee Warren and welterweight Errol Spence. Although three women are still alive, the American men will head home from London with their worst Olympic showing ever.
Warren lost his opening bout in his record third Olympics, falling to France's Nordine Oubaali 19-18. Spence then lost a difficult decision to India's defense-minded Krishan Vikas, falling 13-11 despite showing more aggression throughout the bout.
Welterweight Freddie Evans beat fourth-seeded Egidijus Kavaliauskas of Lithuania 11-7 to run Britain's record to 8-1 at its home Olympics.
Defending gold medalists Todd Rogers and Phil Dalhausser of the United States were knocked out of the Olympics by Italy.
Rogers and Dalhausser lost the first set 21-17 and fell behind Paolo Nicolai and Danielle Lupo 12-7 in the second. The Americans tied it 19-19 but lost the final two points and were eliminated when Rogers' spike was blocked back into him by the 6-foot-8-inch Nicolai.
Jennifer Kessy and April Ross of the U.S. beat Switzerland's Simone Kuhn and Nadine Zumkehr to advance to the quarterfinals of the women's tournament. Brazil's top-seeded Juliana and Larissa also advanced, eliminating the Netherlands 21-10, 21-17, and Laura Ludwig and Sara Goller beat fellow Germans Katrin Holtwick and Ilka Semmler 21-16, 21-15.
Destinee Hooker scored 19 points and the U.S. women's team clinched the top spot in its pool with a preliminary-round victory over Serbia in straight sets.
Logan Tom added 12 points in the 25-17, 25-20, 25-16 sweep. The U.S. will wrap up the preliminary round with a match against Turkey on Sunday.
Brazil stayed alive in the preliminary round with a 3-2 victory over China. Now 2-2, Brazil is still on the ropes with one match left against last-place Serbia on Sunday.
Italy and the Dominican Republic each had three-set victories. Turkey edged South Korea in five, and Russia topped Japan 3-1.
Maggie Steffens scored three goals and the U.S. women's team beat China 7-6 in its final preliminary-stage game.
The U.S. finished the preliminary round even with Spain at the top Group A with five points. But Spain earned the top spot because of the tiebreaker, and the Americans will play 2012 European champion Italy in Sunday's quarterfinals.
Spain beat Hungary 13-11, Australia edged Russia 11-8, and Italy topped host Britain 10-5.
No tears for Victoria Pendleton this time. Just jubilation.
Pendleton washed away the disappointment of her disqualification in the team sprint by winning the Olympic gold medal in the keirin with a flawless performance.
Pendleton, who was devastated and left in tears after the team sprint, raised both arms after crossing the finish line and then brandished a Union Jack to the delight of the 6,000 spectators.
The 31-year-old Pendleton gave Britain its third gold medal in two days of competition on the super fast track, just moments after Ed Clancy, Geraint Thomas, Peter Kennaugh and Steven Burke won the team pursuit title in a world record time.
Mahe Drysdale was really nervous before the men's single sculls final. He felt really great when it was all over.
The star oarsman from New Zealand won his race in the Olympic rowing regatta, completing the resume of one of the sport's leading names. The five-time world champion finished in 6 minutes, 57.82 seconds.
Ondrej Synek of the Czech Republic got the silver, and Britain's Alan Campbell surged late for third.
New Zealand also got a victory in men's pair, with Hamish Bond and Eric Murray extending their three-year unbeaten run.
Katherine Grainger and Anna Watkins won the women's double sculls for host Britain, and Germany upset favorite Croatia for the men's quadruple sculls title.
GYMNASTICS - TRAMPOLINE
China's Dong Dong took the gold in men's competition, putting together a dizzying series of flips and twists. Dmitry Ushakov of Russia was second, and defending Olympic champion Lu Chunlong of China grabbed the bronze.
Charlotte Dujardin's record score riding Valegro helped Britain take a slender lead over Germany after the two-day first round of the dressage competition.
Rafalca, co-owned by the wife of U.S. presidential hopeful Mitt Romney and ridden by Jan Ebeling, scored 70.243 percent to place 30th out of 50 riders.
Wu Minxia and He Zi of China led the preliminaries of women's 3-meter springboard, another dominating performance by the country that captured the first four diving golds of the London Games.
Minxia surged ahead with a brilliant final dive, a back 2½ somersault in the pike position. The crowd gasped when she sliced through the water, barely causing a ripple.
Ben Ainslie won the 10th race in the Finn regatta to set up a dramatic showdown in his quest to become the most successful sailor in Olympic history.
He pulled within two points of Denmark's Jonas Hoegh-Christensen, who has led the Finn fleet since Race 1 and angered Ainslie on Thursday with a claim that the British star hit a turning mark.
Ainslie is trying for his fourth straight gold medal and fifth games medal overall.
China defeated Spain 3-0 to reach the women's quarterfinals, and second-seeded Japan also advanced easily.
Singles gold medalist Li Xiaoxia, silver medalist Ding Ning and Guo Yue got the wins for China, which took the top two spots in women's and men's singles.
Japan also swept the United States.
Hong Kong, Austria, Portugal and Germany won on the men's side.
Argentina earned its first points in the men's field hockey competition, drawing 2-2 with No. 1 Australia.
Argentina captain Matias Vila scored just after halftime and teammate Gonzalo Peillat tied it on a penalty corner in the final minutes.
The Netherlands beat New Zealand 5-1 for its third win in a row. Germany, Britain, Spain and Belgium also won.
Sergei Martynov of Belarus set a world record in the men's 50-meter rifle prone, and Cuba got its first shooting gold medal at an Olympics when Leuris Pupo won the 25-meter rapid fire pistol.
Martynov became the second shooter in Olympic history to score the maximum 600 points in qualification and finished with a total of 705.5 points, beating the 12-year-old mark from Germany's Christian Klees by 0.7.
The silver went to Belgium's Lionel Cox, and Rajmond Debevec of Slovenia took third.
Pupo scored 34 shots in the final, beating Vijay Kumar of India by four shots. Ding Feng of China won bronze with 27 points.
Spain joined France in the women's handball quarterfinals after eliminating Sweden with a 25-24 win, while Croatia and Russia also continued preparations for the next round with victories.
Nely Alberto Francisca and Marta Mangue Gonzalez each scored six goals for Spain.
France, Angola and Norway also won.
ELSEWHERE IN LONDON
Oh Jin-hyek of South Korea won the men's individual archery competition, topping Takaharu Furukawa of Japan in a one-sided final. The bronze went to China's Dai Xiaoxiang, who beat Rick van der Ven of the Netherlands 10-8 in a shoot-off tiebreaker. ... Svetlana Podobedova won Kazakhstan's third weightlifting gold medal of the games, beating Russia's Natalya Zabolotnaya in a women's 75-kilogram showdown decided on the last lift. Poland's Adrian Zielinski made the most defending champion Lu Yong's early exit, winning the men's 85-kg weight class. ... The badminton gold for mixed doubles went to China's Zhang Nan and Zhao Yunlei. ... South Korea won the men's team saber. ... Judo golds went to France's Teddy Riner (over 100-kg) and Cuba's Idalys Ortiz (women's 78-kg).