* Phelps finishes with 18th gold, record 22nd Olympic medal
* Serena wins women's tennis gold
* "Blade Runner" Pistorius makes Games history
* U.S. top medals table ahead of China and Britain (Updates with Phelps's last gold, British cycling record)
LONDON, Aug 4 (Reuters) - The greatest swimmer in history bowed out on a high with gold in the last pool race of the London Olympics as the fastest sprinter in history strutted into the athletics stadium to begin the defence of his 100 metres title.
Michael Phelps swam his favourite butterfly stroke in his farewell race to help the United States to gold in the 4x100 metres medley relay, an event they have never lost.
The medal was his 18th gold in an Olympic career stretching back to Sydney in 2000, and his 22nd of any colour, four more than the previous record held by Soviet gymnast Larisa Latynina.
At the athletics stadium, South African "blade runner" Oscar Pistorius made history as the first double amputee to compete in an Olympic athletic event, coming through his 400 metres heat.
Jamaica's 100 metres world record holder Usain Bolt, the biggest draw of the Games, soaked up the applause of a capacity 80,000 crowd before comfortably winning his heat, albeit with a stuttering start.
His reception was topped only by the cheers for Britain's Jessica Ennis, the poster girl of the London Games, around the track after she clinched heptathlon victory in the final event, the 800m. With heavy national expectations fulfilled, she dropped to the track and burst into tears of relief.
Ennis crowned a day of five golds for the host nation on a hectic middle Saturday of the Games. Britain are now third in the medal table on 13 golds and pushed the volume of support from enthusiastic crowds across the venues to new highs.
The United States moved further ahead of China, leading by 26 golds to 25.
The women's 100m final at the climax of the evening was shaping up as a showdown between Carmelita Jeter of the United States and two Jamaicans, Veronica Campbell-Brown and reigning champion Shelly-Anne Fraser-Pryce, the three fastest qualifiers.
Elsewhere there was a dominant performance from Serena Williams, who crushed Maria Sharapova 6-0 6-1 to win the women's singles tennis title.
Williams needed just 62 minutes on Wimbledon's Centre Court to clinch a career "golden slam" of all four majors and an Olympics singles title, joining Steffi Graf, Andre Agassi and Rafael Nadal.
The Americans cemented their dominance in the pool by winning the women's medley relay in world record time, but had had to share the last-night limelight with China's Sun Yang.
Sun slashed more than three seconds off his own world record to win the men's 1500 metres freestyle to go with his 400m gold.
Ranomi Kromowidjojo of the Netherlands won the women's 50m freestyle to go with her 100m gold and earn the right to be called the fastest woman in the pool.
Britain extended their domination in the velodrome, where their women's pursuit team took gold, breaking the world record for the third time in three races.
The home nation has also been the most successful on the rowing lake, winning two golds on the last day of competition, including a men's four "grudge match" against arch-rivals Australia that had a crowd of 30,000 on its feet.
In London's Hyde Park, the women's triathlon came down to a photo-finish, the first ever in the sport.
After just shy of two hours of swimming, cycling and running, Switzerland's Nicola Spirig was judged to have beaten Lisa Norden of Sweden by just 15 cm (6 inches) after a ferocious sprint finish.
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.
He ran a pedestrian time by his own standards of 10.09 seconds, stumbling slightly at the start, but still qualifying easily for the semi-finals.
"I made a bad step," the 25-year-old said. "I stumbled a bit. I'm glad it happened now."
Also limbering up for Sunday's 100m 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 400m heats, Pistorius raised his arm aloft to acknowledge his 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."
But where there were thrills there were also spills as American defending men's 400m champion LaShawn Merritt, the fastest in the world this year, pulled up injured in his heat.
Former world 100m champion Kim Collins failed to appear in his heat, vowing never to run for St Kitts & Nevis again after falling out with his country's Olympics officials.
Colombian 400m runner Diego Palomeque Echevarria was suspended from the Games after testing positive for testosterone, the fourth athlete suspended by the International Olympic Committee.
And Brazilian rower Kissya Cataldo da Costa has been sent home by her own federation for failing a pre-Games dope test.
"Cheats are being caught and ejected," said IOC spokesman Mark Adams. "At this stage it is a pretty low number."
Ironically, the bans came on a big day for prominent doping offenders Justin Gatlin and Dwain Chambers, who have been allowed to run in London after serving long bans.
"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." (Additional reporting by Karolos Grohmann, Mitch Phillips, Steven Downes and Kate Holton; Editing by Sonya Hepinstall)