PREVIEW-Olympics-Swimming-Phelps and Franklin chase third gold medals

LONDON Aug 2 (Reuters) - Michael Phelps and his American team mate Missy Franklin will both be chasing their third gold medals of the London Olympics on the penultimate day of the swimming competition on Friday.

While the golden pair are looking to their already bulging stockpiles, the host-nation Britain will be looking for a first gold medal in the Aquatics Centre when Rebecca Adlington defends her 800 metres freestyle title.

And the fastest men in the world will square off in the 50 freestyle sprint, a frantic splash and dash that could be decided by a fingernail.

Reinvigorated after winning the 200 medley on Thursday, Phelps will start as the favourite to win the 100 butterfly gold for the third Olympics in succession after setting the fastest qualifying time in the semi-finals.

"It's a 100m, you can't really pace it, you just go for it and hope you hang on," he said.

Despite his domination of Olympic swimming, the challengers keep lining up for Phelps and his opponents in the final include Serbia's Milorad Cavic, who Phelps famously beat by a fingernail in Beijing to win the seventh of his eight golds, and South Africa's Chad le Clos, who beat the American in the 200 final in London and qualified second fastest.

"He (le Clos) can do everything," Phelps said. "You don't see that often."

Adlington, who ended Britain's long drought in swimming by winning two gold medals in Beijing four years ago, stayed on course to defend her 800 title after setting the fastest time on Thursday's heats.

But she also faces some tough opponents, most notable Denmark's former world champion Lotte Friis and 15-year-old Katie Kedecky, the youngest member on the powerful American team.

"I'm going to give it my all in the final, it's all about who can get that finish," Adlington said.

Franklin qualified second fastest behind her team mate Elizabeth Beisel for the 200 backstroke final but swam conservatively because her semi-final was held just before she competed in the 100 freestyle final.

"I have a lot of confidence going into it," Franklin said.

"It's my favourite race and I have so much fun with it, that's the most important part."

The 50 freestyle final is looming as anyone's race after the eight finalists qualified within a third of a second of each other with Brazil's defending champion Cesar Cielo and American Cullen Jones dead-heating for first in the semi-finals. (Editing by Greg Stutchbury)