The football tournament at the Beijing [Images] Olympics [Images] bursts into life on Tuesday with a semi-final between two of the sport's fiercest rivals, Brazil [Images] and Argentina, in a contest enlivened by the genius of Ronaldinho [Images] and Lionel Messi [Images] on opposing sides.

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Brazil are hoping for revenge against holders Argentina for defeat in Athens.

Argentina's brilliant performance at the 2004 Olympics had won them a gold medal.

On the contrary, five-time World Cup champions Brazil, 'the kingdom of football', has yet to claim an Olympic medal to match its status as the world's biggest football powerhouse. The missing 'Olympics' link will motivate the Brazilian team to fight hard for glory.

"Brazil have a lot of respect for Argentina and we respect them too," Argentine Sergio Batista said, adding, "I hope we can see an open match. Both teams have players with enough quality to make a beautiful match."

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Ronaldinho is one of three over-age players in a team who otherwise have to be under 23 at the start of the year.

The AC Milan player is being bombarded with autograph requests in the Olympic Village, forcing Brazilian players to surround him at times.

Another semifinal is between two Olympic champions: Belgium and Nigeria.

As the Antwerp 1920 Olympic football champion, Belgium made a stunning comeback after 80 years' absence from the Olympic football competition by upsetting favorite Italy [Images] 3-2 in the quarterfinal.

Nigeria, the Atlanta 1996 Olympic champion, is never short of gifted footballers.

Having performed impressively in the tournament, the African Eagle will take off to rebuild its Olympic glory. But the team's typically unstable performance on the international football scene adds to the uncertainty of the match's outcome.

Photographs: Getty Images