MANAUS Brazil (Reuters) - Germany marked their 100th World Cup game in style on Monday by thumping shambolic Portugal 4-0 in the midday sun to confirm their billing as title contenders while the United States snatched a win over Ghana.

The tournament continued to live up to its reputation as one of the best in recent years and in a hugely enjoyable, fast-paced game the U.S. grabbed a late goal for a surprise 2-1 victory over their deflated opponents.

Earlier, African champions Nigeria could only stumble to a dire goalless draw with Iran in the first really poor game spectators have seen in Brazil.

Germany became the first team to notch up a century of World Cup games and Thomas Mueller's hat-trick - the first of the tournament - easily saw off of Portugal in a 4-0 win in their Group G opener in Salvador.

The occasion had been billed as clash between two European heavyweights but was effectively over in the opening 45 minutes when the Germans scored three times and combative Portuguese center back Pepe was sent off.

In the second half, Germany played at a much slower pace as the stupefying heat took its toll but still scored a fourth goal through Mueller.

"It was incredibly hot on the pitch. It's nice to get a great win like this today but we've still got a long way to go," Germany captain Philipp Lahm said.

Portugal coach Paulo Bento suggested the referee was biased in favor of Germany but his players let him down badly.

World player of the year Cristiano Ronaldo had a disappointing game and there must be doubts now about the prospects for Portugal, who next face a buoyant United States on Sunday in Manaus.

In the other Group G match, Clint Dempsey scored after 29 seconds in Natal, the fifth fastest goal in World Cup history, and the Americans then faced a Ghanaian onslaught for the rest of the game.

After a string of goalmouth scrambles, near misses and fine saves by U.S. keeper Tim Howard, Andre Ayew equalized in the 82nd minute. But four minutes later U.S. substitute John Brooks headed in from a corner for his first international goal.

"There are things we can improve on but we got the three points we badly wanted," U.S. coach Juergen Klinsmann said.

"Ghana is a good team, they know how to grind the game. It was what we expected."

The victory represented revenge over the team who had ended U.S. participation in the last two World Cups. It also meant Klinsmann had made a winning start in all five World Cups he has been involved in as coach and player.

SHEER EXCITEMENT

The sheer excitement of this tournament - which contrasts painfully with the many boring games in South Africa in 2010 - has fans buzzing.

"The World Cup has been dour and too tactical for years and this has been wide open," said England fan Ross Letham as he watched a game on a big screen in Sao Paulo.

Brazil attacking midfielder Zico said he believed coaches were largely to thank.

"I think the coaches have adopted the philosophy of first scoring and then defending," he told a FIFA media briefing.

Just to remind fans what they had not been missing, Iran and Nigeria served up an appalling draw in Curitiba. Neither side seemed able to pass the ball or shoot on target and it was Iran who had the best chance to score.

Brazilian supporters will be expecting much better when their side take on Mexico in their second Group A match in Fortaleza on Tuesday. Both teams won their opening games.

Brazil will keep Mexico guessing until the very last minute as to whether they will face burly forward Hulk, coach Luiz Felipe Scolari indicated. Hulk has been trying to shake off a thigh injury. The Mexicans plan to field the same starting 11.

Also on Tuesday, Group H becomes the last to get into action, with highly-fancied Belgium facing Algeria and Russia meeting South Korea.

The Russian players have said little of interest so far and their Italian coach Fabio Capello, a renowned disciplinarian, said on Monday he had banned the squad from using Twitter because it could be a nuisance.

Brazil suffered from major technical and infrastructure challenges in the run-up to the World Cup, one of them being a lack of accommodation.

In the jungle city of Manaus, some tourists were forced to venture into the red light district and stay in "love hotels", which usually rent out their rooms for a couple of hours to amorous couples.

"It was either sleep in the street or sleep in the love hotel. And we have been propositioned every night," said England fan Marc Cummings with a laugh.