By Mark Trevelyan

LONDON (Reuters) - An Australian, an American and a Brazilian vie for the title of world's fastest swimmer in the men's Olympic 100 meters freestyle final on Wednesday, a day after Michael Phelps won a record 19th Games medal.

James Magnussen, Nathan Adrian and Cesar Cielo, the world record holder, will thrash it out in the Blue Riband event of the swimming competition which has provided the outstanding highlights of London 2012's first four days.

Two medals in the pool on Tuesday night enabled Phelps to first match and then overhaul the total of 18 held for nearly half a century by Soviet gymnast Larisa Latynina.

The American picked up silver in the 200 butterfly, his favorite event, when he led going into the last few meters but messed up his final touch to allow South Africa's Chad Le Clos to snatch victory by his fingertips.

An hour later, the 27-year-old Phelps won the right to call himself the greatest Olympian of all time when the U.S. team destroyed their rivals in the 4x200 freestyle relay.

Swimming the anchor leg, he cruised home to clinch his 19th medal - a 15th gold to go with two silvers and two bronzes.

Phelps saluted his team mates who flung their arms around him. "I thank those guys for helping me get to this moment," he said.

It was an historic moment in the 116-year annals of the modern Olympic Games. Latynina, 77, who won nine golds, five silvers and four bronzes between 1956-64, was among the crowd who rose to their feet in the 17,500-capacity Aquatics Centre.

Minutes earlier Chinese prodigy Ye Shiwen won her second gold of the Games, setting an Olympic record in the 200 individual medley having stunned swimming pundits with her victory and world record in the 400 medley three days earlier.

The 16-year-old, who swam her last length on Saturday faster than Phelps's team mate Ryan Lochte did in winning the corresponding men's event, has been forced to fend off insinuations of doping - not backed by any evidence - that drew a sharp response from Chinese officials.

"Ye Shiwen has been seen as a genius since she was young and her performance vindicates that," Xu Qi, head of the Chinese swimming team, told the news agency Xinhua.

"Don't use your own suspicions to knock down others. This shows lack of respect for athletes and for Chinese swimming."

CHINA TOPS TABLE

Elsewhere on Day Four, Germany won their first two golds, in individual and team eventing, and France grabbed their fourth, in canoe slalom.

The U.S. women's team stormed to the gymnastics gold - the country's first since 1996 - with dazzling performances from Jordyn Wieber and Gabby Douglas.

China top the medals table with 13 golds followed by the United States with nine. Each has 23 medals in all.

Hosts Britain, who dazzled at Beijing in 2008 to take fourth place with 19 golds, are still awaiting their first in London.

They have high hopes on Wednesday morning when Helen Glover and Heather Stanning compete in the pairs rowing, an event the duo have dominated this season and set a new Olympic best in their heat.

Bradley Wiggins, the first Briton to win the Tour de France, will also aim to land his fourth Olympic gold when he rides in the individual cycling time trial at Hampton Court Palace outside London.

Ten days after completing the Tour, he must lift himself once again following another exhausting effort on Saturday when he toiled at the front of the peloton in a desperate but futile effort by the British team to set up Mark Cavendish for gold in the Olympic road race.

Medals are also up for grabs on Wednesday in table tennis, fencing, judo, shooting, diving, canoe slalom, weightlifting and gymnastics.

Triple Olympic gymnastics champion Kohei Uchimura, who helped Japan win team silver on Monday after an official protest over his marks, is favorite to win the all-around gold.

(Editing by Tony Jimenez)