By Pritha Sarkar

BEIJING (Reuters) - The Americans began their post-Hamm era by setting the early pace in men's qualifying on the opening day of the gymnastics competition on Saturday.

Twins Morgan and Paul Hamm, the 2004 all-round Olympic champion, had led the team at the last two Olympics but following their withdrawals through injury, it was left to Jonathan Horton and Justin Spring to show the way.

With title favorites China and Athens Games gold medalists Japan due to begin their much-anticipated showdown later on Saturday, the U.S. made sure they did not suffer any major mishaps as they aimed to reach Tuesday's eight-team final.

Under the new open-ended scoring system, making its Olympic debut following the scrapping of perfect 10.00 system, the 2004 silver medalists easily eclipsed Spain with a total of 365.200 with their solid performances. Italy were a distant third.

Horton led the all-round hunt at the National Indoor Stadium with a combined total of 91.650, ahead of Spain's Rafael Martinez who trailed by 0.85 of a point.

Spain's Sergio Munoz suffered the most dramatic fall in the first subdivision when he flew off the horizontal bar mid-routine and landed on his head.

His score of 13.300 dented his team's hopes of reaching the final since he and team mate Manuel Carballo were rewarded with the two lowest scores among all the competitors.

Olympic champion Igor Cassina, of Italy, showed his rivals how it should be done and his gravity-defying routine on the bar made him the top qualifier for the apparatus final with 16.000.

(Editing by Alison Williams)