By Pritha Sarkar

RIO DE JANEIRO (Reuters) - Kohei Uchimura's clumsy slip on the horizontal bar not only shocked fans at the Rio Olympic Arena on Saturday but came to represent a horror day for Japan's men's gymnastics team in their efforts to dethrone China as Olympic champions.

Uchimura, who has won six all around world titles and an Olympic gold, produced a deafening thud after he lost his grip and fell into an ungainly heap on the floor.

Although the 27-year-old got back on the bar to earn 14.300, the fall summed up a poor day of men's qualifying for Japan, who featured four survivors from their 2012 silver-medal winning squad but were upstaged by their Asian rivals, the United States and Russia on the opening day of competition.

As the reigning world champions, they would have been expected to breach the 15-point mark in many of their performances but instead all they did was produce a succession of botched routines.

Yusuke Tanaka was the first to stumble with 13.366 on the pommel horse, before Ryohei Kato earned 13.966 on rings and Koji Yamamuro sat down and almost rolled off the crash mat following his vault to score 13.200.

Yamamuro (12.733) and Tanaka (13.866) both failed to impress on the parallel bars, while Uchimura's mishap was particularly surprising considering he is the world champion on the apparatus.

It was not until they performed on their final apparatus, the floor exercise, that all four Japanese gymnasts scored over 15 points on the same discipline.

The 19-year-old Kenzo Shirai was the only one who did not let the team down but he only performed on the vault and floor.

"I don't think the mistake on the high bar was a bad thing because... what happened today can only make me stronger," said Uchimura, whose fall meant he was out of the running to win the horizontal bar title in Rio.

"We try and perform like world champions but this is the Olympic Games and it stresses us out."

Luckily for Japan none of the scores will be carried through to Monday's team final but what they do know is that if they want to end China's eight-year reign as Olympic champions, they cannot afford another similar performance.

Uchimura can, however, take heart from the fact he won the 2012 all around gold despite finishing ninth in qualifying and remains the favorite to become the first man since compatriot Sawao Kato in 1972 to win successive Olympic all around titles.

(This version of the story refiles earlier verison)

(Editing by Greg Stutchbury)