By Anthony Boadle

RIO DE JANEIRO (Reuters) - A less experienced U.S. men's volleyball team demolished Poland in straight sets on Wednesday and moved up to the semifinals looking to avenge early upsets that almost knocked the Americans out of the Rio Games.

Brazil ousted their sport archrivals Argentina in a four-set battle in front of an ecstatic and boisterous home crowd of 10,000 at Rio's Maracanazinho stadium.

With eight of their 12 players competing in the Olympics for the first time, the United States outplayed Poland, 25-23 25-22 25-20, ousting a team that won the 2014 World Championship.

The U.S. team, formed from a crop of barely graduated college players, got off to a shaky start in Rio losing their first two games to Canada and Italy.

But they learnt fast from their mistakes and came back with a stunning win over hosts Brazil and then overpowered France to stay in Rio.

The United States will get a second crack at Italy in the semi-finals on Friday after the Italians defeated a strong Iranian team in three sets.

"I want them bad. We want a little revenge," said wing spiker Taylor Sander, among the top three U.S. scorers.

The increasingly confident Americans came into the game against Poland more aggressively than ever and appear to be reaching their best form as they enter the medal-winning phase.

"We are so aggressive, our blocks are in very good spots and defensively we are good because we are serving the ball hard," Sander told Reuters after the game.

U.S. coach John Speraw said his team were on a fast learning curve and responded immediately to some "deep" talks on their game that had given them new direction and "incredible" energy.

"We are all so new. With a team as young as they are, I had a hunch that, if we were going to reach our very best, it was going to take some adversity in this tournament," he said.

"We have made significant strides in our game and we are playing really hard," Speraw told a news conference.

"Our best is ahead of us."

Italy coach Gianlorenzo Blengini acknowledged that the United States had grown in the tournament after a bad start and would be a stronger adversary on Friday.

"They are a great team that has overcome their complicated beginning with much serenity," Blengini told reporters.

Iran lost four set points in an exciting first set and went down to Italy 31-29 25-19 25-17.

Current Olympic champions Russia eliminated Canada 3-0 and go on to face Brazil who overpowered Argentina 25-22 17-25 25-19 25-23.

Brazilian fans booed at every Argentine serve and erupted in cheers when the home team scored.

Borrowing a chant from their bitter soccer rivalry with Argentina, they sang "1,000 goals, only Pele scored 1,000 goals! Maradona is a cocaine snorter."

(Editing by Andrew Hay/Sudipto Ganguly)