HANOI, Vietnam _ Japan and Qatar play their opening Asian Cup match Monday knowing that their respective moments of truth will only come later in the tournament.

Japan, the two-time defending champion, is expected to have few problems negotiating Group B against Qatar, hosts Vietnam and Gulf Cup champion United Arab Emirates.

Instead they could be forgiven for thinking about later clashes against genuine title rivals such as Australia, Iran and South Korea in the knock-out stages.

However, coach Ivica Osim tried to downplay expectations of Japanese fans anticipating a third consecutive title.

"If we are champions of the Asian Cup forever, it will not be good for Asian football," Osim told reporters in Hanoi, adding that his team had prepared well to play against the Asian Games champion Qatar.

"All the games will be difficult, but our national team has its best players now," said Osim.

Japan had its fi squad was ready for their first challenge in the tournament.

"We are not afraid of Japanese team," Qatar captain Alshammari Saad said. "We will try our best to qualify for the next round."