(Adds Sebastian Coe quotes, tennis details)
* Serena wins women's tennis gold
* "Blade Runner" Pistorius makes Games history
* Phelps joins U.S. team in last pool action
* U.S. top medals table ahead of China, Britain
LONDON, Aug 4 The world's fastest man Usain Bolt burst into the Olympics on an action-packed "Super Saturday", cantering to victory in a qualifying race amid roars from a capacity 80,000 crowd at London's main stadium.
The towering Jamaican, the biggest draw of the Games, pointed to the sky as he soaked up the applause before winning his 100 metres heat in what looked like a comfortable stroll under bright sunshine, allaying doubts about his fitness.
Earlier on the eighth full day of competition, South African "blade runner" Oscar Pistorius made history as the first double amputee to compete at the Olympics in a 400 heat which he came through successfully.
The focus was firmly on the track and field after swimming dominated an opening week marked by enthusiastic crowds and the home nation racing up the medals table.
"Good Vibrations" by The Beach Boys blared at the athletics, summing up the mood as the middle weekend began.
In the pool, Michael Phelps has one last chance to add to the biggest Olympic medal haul of all time, while 25 medals in events ranging from tennis at Wimbledon to rowing near Windsor were being decided.
Phelps's heroics and Serena Williams demolishing Maria Sharapova 6-0 6-1 to win the women's singles tennis put the United States at the head of the overall medals ranking with 23 golds, overtaking China, on 22, for the first time.
Host nation Britain moved into third with two rowing golds, including the men's four "grudge match" against arch-rivals Australia that had an excited crowd of 30,000 on its feet.
In athletics, Bolt aims to repeat the heroics of Beijing in 2008 where he won three golds and lit up the world with his lightning bolt celebrations.
Also limbering up for Sunday's 100 final, the most prestigious event of the Games, were the three other fastest men in history - fellow Jamaicans Yohan Blake and Asafa Powell and Tyson Gay of the United States.
In the men's 400 heats, Pistorius raised his arm aloft to acknowledge the warm reception. Born without a fibula in both legs, he fought for the right to line up against able-bodied competition, racing in his carbon fibre prosthetic blades.
"I was so nervous this morning," an elated Pistorius told reporters. "Thanks to everyone for showing their support. I didn't know whether to cry."
Bolt assured those concerned about his fitness with a first-round run of 10.09 seconds, although he stumbled slightly at the start before settling into his powerful stride.
"I made a bad step," the 25-year-old said. "I stumbled a bit. I'm glad it happened now."
Former world champion Kim Collins failed to appear in his heat, however, having vowed never to run for St Kitts & Nevis again following an apparent fallout with his country's Olympics officials.
Home favourite Jessica Ennis, the poster girl of the London Olympics, kept up her pursuit of gold in the heptathlon by extending her overnight lead after the long jump and javelin.
With her every move met by deafening cheers, the 26-year-old has one discipline to go - the 800.
But where there were thrills there were also spills as American defending men's 400 champion LaShawn Merritt, the fastest in the world this year with 44.12 seconds, pulled up injured in his qualifying heat.
Later on Saturday the women sprinters do battle in the 100 final with American Carmelita Jeter favourite, but defending champion Shelly-Ann Fraser and her fellow Jamaican Veronica Campbell-Brown will be out to keep the title in the Caribbean.
Away from the sporting action, Colombian 400 runner Diego Palomeque Echevarria was temporarily banned from the Games following a positive test for testosterone, the International Olympic Committee said.
Brazilian female rower Kissya Cataldo da Costa has also been expelled from the Olympics for failing a dope test for EPO prior to the start of the official Olympic period, a Brazilian rowing team official added.
The 18-year-old Colombian is the fourth athlete to have been officially suspended, provisionally or permanently, from the Games since the start of the Olympic period on July 16.
Ironically, the bans came on a big day for former prominent doping offenders Justin Gatlin and Dwain Chambers, who appeared in the athletics.
"I would clearly rather have that these competitors are not here," London Games chief Sebastian Coe told Reuters.
"The federation says they are eligible to compete, the IOC says they are eligible to compete so we give them as much courtesy as all the other athletes. The answer is that that is the world we live in."
The last night of swimming action sees Phelps join his American team mates in the 4x100 medley final as he looks for his 22nd Olympic medal before retiring.
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 butterfly.
On the water at Dorney Lake outside London, there were two British golds in 20 minutes including that of the men's four, billed as the race of the Olympic rowing competition.
The home crew recorded Britain's fourth consecutive win in the discipline and the host nation's medal tally on the course of nine makes it the most successful Olympic rowing regatta for the country in the modern era.
Joy was tempered, however, when the Danish boat of Mads Rasmussen and Rasmus Quist pipped the British defending champions in the lightweight double men's scull at the death.
Among the other medals decided on a hectic day was a thrilling women's triathlon that went down to a photo-finish, the first ever in the sport.
After just shy of two hours of racing, Swiss Nicola Spirig was judged to have beaten Lisa Norden of Sweden by 15 centimetres after a ferocious sprint finish in Hyde Park in central London.
Williams blew away Russian Sharapova in just 62 minutes on Wimbledon's Centre Court, completing the "golden slam" of all four majors and an Olympics singles title. (Additional reporting by Karolos Grohmann, Mitch Phillips, Steven Downes and Kate Holton; Editing by Alison Williams)
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