By Mark Trevelyan
LONDON (Reuters) - Ethiopia's Tirunesh Dibaba powered away from two Kenyan rivals to capture the first track gold of the London Olympics in the women's 10,000 meters on Friday, while three wins in the pool propelled the United States to the top of the medals table.
Dibaba, the defending champion, surged clear at the bell and strode out to beat Sally Kipyego by about 30 meters, with world champion Vivian Cheruiyot taking the bronze.
Poland's Tomasz Majewski, also a victor in Beijing four years ago, hurled 21.89 meters to beat world champion David Storl of Germany by three centimeters in the shot put.
With flashbulbs popping, music blaring and 80,000 fans creating a deafening roar on the first night of action in the Olympic Stadium, British favorite Jessica Ennis captured the lead in a see-saw heptathlon contest and world champion Carmelita Jeter of the United States ran the fastest time in the women's 100 meter heats.
For the first time at London 2012, the United States overhauled China to lead the medals table by 21 golds to 20.
Michael Phelps, the most decorated Olympian of all time who has said he will retire after the Games, won his 21st medal by claiming the men's 100 meters butterfly.
"This is my last individual event. It was awesome," Phelps said. "This swim was pretty important to me. I wanted to win."
Katie Ledecky, 15, took the women's 800m freestyle title and another teenage U.S. swimmer, 17-year-old Missy Franklin, grabbed her third gold medal of the Games in the 200m backstroke, breaking the world record in the process.
Older competitors also had their day.
Sergei Martynov, a 44-year-old Belarussian, used a 13-year-old gun and bullets from the Soviet era to win the men's 50 meters prone rifle with a world record score.
With the start of the athletics, the jewel in the Olympic crown, excitement began to build towards Jamaican Usain Bolt's defence of his 100 meters title on Sunday and his 200 crown four days later.
Ennis, Britain's Olympic poster girl, set a world best time for a heptathlete in the 100 meters hurdles and followed with a solid high jump, but Lithuania's Austra Skujyte bettered her by more than 3 meters in the shot put to take the lead.
Urged on by the crowd, Ennis overhauled her in the last heptathlon event of the day, the 200m, with a time of 22.83 seconds compared with the Lithuanian's 25.43.
She takes a lead of 184 points into the second day of the event, which concludes with the long jump, javelin and 800m.
Jeter made a big statement of intent in the heats of the women's 100 meters, powering home in 10.83 seconds to set the fourth fastest time of the year.
Defending champion Shelly-Ann Fraser-Pryce of Jamaica looked in no mood to try to match Jeter but qualified comfortably, as did her compatriot Veronica Campbell-Brown and Allyson Felix, of the United States.
At Wimbledon, Roger Federer of Switzerland remained on course to repeat his heroics in the Grand Slam in July, beating Argentina's Juan Martin del Potro in a marathon 19-17 third and final set for a place in the final.
Federer will now face local hope Andy Murray who rode a wave of British euphoria to beat Serbia's world number two Novak Djokovic 7-5 7-5 and set up a repeat of last month's Wimbledon final against the Swiss maestro.
New Zealand struck gold twice on the water, with Mahe Drysdale taking the men's single sculls and men's pair Eric Murray and Hamish Bond cruising to victory in comprehensive fashion.
Britain's Katherine Grainger and Anna Watkins triumphed before 25,000 ecstatic fans at Dorney Lake in the women's double sculls - a dream come true for Grainger after three previous silvers.
Germany powered to gold in the men's quad sculls, finally getting their revenge on the young Croatian crew who had beaten them all season.
But another German rower, Nadja Drygalla, who has already finished competing at the Games, voluntarily left the Olympic village following reports that her boyfriend was a far-right extremist.
In cycling, the British men beat Australia in the team pursuit, setting a world record, and Victoria Pendleton won the women's keirin, a day after being disqualified with Jessica Varnish in the team sprint.
Britain's men took cycling track team sprint gold on Thursday, but the taste of victory was soured when German-born rider Philip Hindes admitted to falling over on his bike on purpose in the heats to avoid being disqualified.
"I did it on purpose to get a restart ... it was all planned really," he told reporters, prompting shock among British media and leading to calls for a change in the rules.
It also raised uncomfortable questions about gamesmanship at the Olympics, after eight badminton players were thrown out for deliberately losing matches to manipulate the draw.
British cycling officials later said Hindes' comments were lost in translation and the International Olympic Committee has no plans to investigate the incident "at present".
(Additional reporting by Kate Holton, Annika Breidthardt, Mark Trevelyan and Julien Pretot; Editing by Alison Williams)