(Reuters) - The world's fastest man Usain Bolt begins the defence of his 100 metres title at London 2012 on Saturday and Michael Phelps will have one last chance in the pool to add to the biggest Olympic medal haul of all time.
Bolt, the charismatic Jamaican who lit up the 2008 Beijing Games with three golds and his trademark lightning bolt celebration, will run in the heats of the shorter sprint, the most eagerly anticipated event of the Games.
Also limbering up for Sunday's final will be the three other fastest men in history, all intent on toppling him: fellow Jamaicans Yohan Blake and Asafa Powell, and Tyson Gay of the United States.
Oscar Pistorius, South Africa's "Blade Runner", will become the first double amputee to compete in the Olympics when he lines up in heat one of the 400 metres.
Born without a fibula in both legs, Pistorius fought for the right to line up against able-bodied competition, racing in his carbon fibre prosthetic blades.
The last night of swimming action will see Phelps join his American team mates in the 4x100 metres medley final and looking for his 22nd Olympic medal.
Phelps, who this week smashed the previous record of 18 that Soviet gymnast Larisa Latynina had held for nearly half a century, notched up number 21 on Friday with gold in the 100 metres 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.
"It hurt so bad in the last 25, that's the part that I love, knowing that I'm pushing myself past the limit," Franklin said.
She too will be looking for a medley relay gold to set the seal on her first Olympics.
Their feats propelled the United States to the top of the overall medals table for the first time, leading China by 21 golds to 20, with South Korea third on nine.
On Friday's first day of athletics action in the Olympic Stadium, Ethiopia's Tirunesh Dibaba grabbed gold in the women's 10,000 metres by kicking fiercely at the bell to shake off two Kenyan rivals.
Dibaba, the defending champion, strode out to beat Sally Kipyego by some 30 metres, with world champion Vivian Cheruiyot taking the bronze.
Poland's Tomasz Majewski became the first man for 56 years to win back-to-back Olympic shot put titles by hurling 21.89 metres to beat world champion David Storl of Germany by three centimetres.
With cameras flashing, music blaring and 80,000 fans creating a deafening roar, British favourite Jessica Ennis captured the lead in a see-saw heptathlon contest.
Ennis, Britain's Olympic poster girl, set a world best time for a heptathlete in the 100 metres hurdles and followed with a solid high jump, but Lithuania's Austra Skujyte bettered her by more than three metres 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 her rival'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.
World champion Carmelita Jeter of the United States ran the fastest time in the women's 100 metre heats ahead of Saturday's semi-finals and final, sprinting home in 10.83 seconds.
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 grasscourt grand slam in July, beating Argentina's Juan Martin del Potro in a marathon 19-17 third and final set to earn 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.
Hosts Britain celebrated wins for Katherine Grainger and Anna Watkins in double sculls rowing - a dream come true for Grainger after three previous silvers - and for their men's team pursuit cyclists and Victoria Pendleton in the women's keirin.
Britain demolished Australia and their own world record to defend the men's Olympic team pursuit gold medal they won four years ago in Beijing and send the home crowd into a state of delirium once again in the velodrome on Friday.
David Bowie's "Heroes" blasted out over the tannoy as the home fans clapped the gold medallists on a lap of honour after they shaved almost a second off the previous record.
Britain are in fourth place in the medals table with eight golds - the same number as France, who celebrated wins for Teddy Riner in judo and swimmer Florent Manaudou in the 50 metres freestyle.
A light lunch made the difference between gold and silver for Polish weightlifter Adrian Edward Zielinski.
Tied with Apti Aukhadov from Russia after they lifted the same total weight, he won on lighter bodyweight.
"I ate a whole breakfast: boiled eggs and a little bit of porridge and for lunch I just had a small piece of chicken," said the Polish gold medallist who weighed in 130 grams lighter than his opponent.
(Editing by Alison Williams)