By Martyn Herman

BEIJING (Reuters) - A golden showdown between Roger Federer and Rafael Nadal looms on the murky Beijing horizon as the Olympic tennis tournament begins on Sunday.

Swiss Federer, the dominant force in men's tennis for the past five years, leads a grand slam quality field with serious questions to answer over his flagging form.

Nadal, who will replace Federer as the new world number one the day after the men's final whatever happens here, has no such worries, although a grueling year may yet catch up with him in the sauna-like heat of Beijing.

Serbia's rise as a force in tennis is underlined by men's third seed Novak Djokovic and especially in the women's singles, where Ana Ivanovic and Jelena Jankovic, who takes over as world number one on Monday, top the seedings.

Russia are also well-served despite the absence of Maria Sharapova, with world number three Svetlana Kuznetsova their best hope on her favorite hardcourt surface.

Eastern Europeans swamp the draw, but many predict American sisters Venus and Serena Williams will go home with gold in their racket bags.

"I love playing for my country," fourth seed Serena, a doubles winner with her sister in Sydney in 2000, where Venus also won the singles, told reporters on Friday.

"Of all my trophies, my favourite is my Olympic gold medal. It was a life-changing experience."

TAKING IT SERIOUSLY

Twenty years after the sport returned to the Games in Seoul amid some mild skepticism, there is no longer any doubt how seriously the players take the Olympics.

Germany's Rainer Schuettler even went to the Court of Arbitration for Sport (CAS) this week in his desperation to grab a late place in the 64-strong draw after falling short of the stringent ranking cut-off point in June. His appeal was successful, but the row with the International Tennis Federation has rumbled on all week.

Federer, who failed to make an impact in Sydney or Athens, has long set his sights on an Olympic gold medal to add to the 12 grand slam titles he has already won.

The Swiss, who turned 27 on the day of the opening ceremony, has been working overtime in practice all week in stifling humidity, searching for the spark that could yet turn his year round after a gut-wrenching defeat by Nadal at Wimbledon.

While containing the powerhouse Majorcan is proving difficult for Federer, the thought of meeting him in a gold medal match is enticing.

"Our rivalry is great for tennis right now. I think we've played each other 16 times and given our ages ... we will probably play another 20 times," he said.

"We are very different characters but it is on the fair play side. I think we will leave our mark on the sport."

In the doubles events American twins Bob and Mike Bryan are seeded one while compatriots Lindsay Davenport and Liezel Huber offer hope of more American gold in the women's.

(Editing by Alex Richardson)