"We have fire because everyone on this team is very young.
Everybody wants to play with emotion," spiker Tsvetan Sokolov said after Bulgaria's 32-30 25-20 25-19 win at Earls Court.
The team finished fifth in Beijing four years ago but were expected to struggle to match that achievement in London following a difficult build-up that included the departure of coach Radostin Stoychev in June.
"Nobody expected we would (perform so well) but we have shown the world that no one can mess with us," spiker Todor Aleksiev said.
Bulgaria now play Germany in the quarter-finals while Italy face a tough task against Beijing champions and Group B winners the United States.
The top finishers in each group face the fourth-placed team from the other section and there is a draw to determine the other quarter-finals.
The US scored a straightforward 25-15 25-19 25-19 victory over winless Tunisia.
Beijing runners-up Brazil and bronze medallists Russia also qualified for the last eight from Group B along with outsiders Germany.
"USA are a team playing well but it's OK (to be playing them), any team would be a strong team now," Italian captain Cristian Savani said.
Bulgaria had another young team to thank for helping them win Group A as fifth-placed Australia recorded a surprise 25-21 25-22 18-25 25-22 win over Poland who finished second.
"It has been an amazing tournament for us. It is clear how the team have evolved day-by-day," Australia coach Jon Uriate said after the team rebounded from 3-0 whitewashes by Argentina and Bulgaria to almost topple Italy and then defeat Poland.
"I think we showed against Italy that we could win against a top team and this group (of players) is very smart," added Uriate who won a bronze medal as a player for Argentina in the 1988 Seoul Olympics.
Uriate became Australia's coach in May 2011 and has rebuilt the team following the retirement of many senior players at the end of the 2010-11 season.
"The evolution in 13 months has been huge," he said, although his team failed to qualify for the quarter-finals.
Argentina progressed with a 25-18 25-18 25-15 win over a British team that has failed to win a set in the tournament.
"We're very tired. We've emotionally flat. I've tried to push them. It's been our first big tournament so there's been a lot of emotional strength out of our body," said Britain coach Harry Brokking.
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