If Japan, South Korea or Australia are to win the Asian Cup in Qatar they will have to do it the hard way in the face of hostile crowds roaring on their rivals from the Middle East.

Most in a crowd of nearly 40,000 were behind Iraq in their shock 2-1 win over four-time champions Japan, a day before South Korea wilted under pressure from Jordan before salvaging a last-gasp 2-2 draw.

Australia, another of the title contenders, have won both matches so far but their players said that facing Syria in Doha had been like an away game.

Japan coach Hajime Moriyasu said that Iraq's vocal fans had played a part in their team's stunning triumph over the pre-tournament favourites.

"We expected that the atmosphere in the stadium would give Iraq energy and that they would pressure us, and the aim was to stop that," he said.

"But they came at us harder than we expected."

It was a similar story for Jurgen Klinsmann's South Korea a day later, with the vast majority of the 36,000 fans at Al Thumama Stadium backing Jordan.

Like Iraq against Japan, Jordan hit South Korea with a wave of relentless energy, and were unlucky not to claim a famous win.

Klinsmann said it was "a very intense game" but he believes it will help his team as they attempt to win the tournament for the first time since 1960.

"I think today was very good for us to experience the emotional side and the fighting side from Jordan," said the German, a World Cup winner as a player.

"I am sure that throughout this tournament we will face more Arabic nations in the knockout phase.

"It is important for us to take every lesson in the group phase."

The compact Jassim bin Hamad Stadium, where Australia squeezed past Syria 1-0, was not used at the 2022 World Cup and has a comparatively small capacity of just over 12,000.

Even though it was not full, the Syrian fans made a racket every time their side threatened. In contrast, there was just a few small pockets of Australian supporters.

"Any time they get in the box or there's even a half-chance, the crowd get up and it feels more of a chance than it actually is," defender Harry Souttar said after the Socceroos won with their only shot on target.

"It's just about remaining calm and thinking it wasn't actually as dangerous as everyone thinks it is and made it out to be."

A crowd of 82,490 watched hosts and holders Qatar beat Lebanon 3-0 in a record attendance for an Asian Cup opening game.

Decent crowds are good news for organisers and countries from the immediate region -- but bad for the rest of the teams.

Australia coach Graham Arnold said playing in front of noisy partisan crowds was a good experience for the younger members of his squad.

"For them it was probably the first time that they experienced playing in the Middle East and in front of a crowd like that -- so one-sided," he said.

He added: "We knew that with the stadium here, with a lot of the crowd supporting them, that Syria would make it difficult for us."