On a day of samba, surprises and Russian firepower at Earls Court, the hitherto underperforming Italians conjured an inspired display out of nowhere to beat the US 3-0.
In the semi-finals they will face top-ranked Brazil who fed off a carnival atmosphere to crush south American rivals Argentina.
By far the loudest noise of the day came later, however, when Poland, roared on by legions of fans in white and red, lost 3-0 to arch-rivals Russia in an electrifying atmosphere in their last eight match.
With chants of "Polska, Polska" ringing around the old London arena, reminiscent of the Euro 2012 championship co-hosted by Poland, Russia's own "poles" were unshakable in a clash between the world's second and third ranked nations.
Dmitry Muserskiy, all 2.18 metres of him, and Maxim Mikhaylov, a mere 2.02m, were the twin towers on which Russia's victory was built - scoring 25 of their side's points and blocking everything Poland threw at them.
Russia will play Bulgaria for a place in their first Olympic final since the Soviet Union won gold in Moscow in 1980.
Bulgaria celebrated as if they had won the gold medal when outclassing Germany 3-0 in the day's last game.
After Brazil earned the South American bragging rights by dispatching an inexperienced Argentina side with relative ease, they would have expected a second clash with the United States after losing to them in group play.
However, Italy had not read the script.
After saving two set points in the opener the Azzurri turned on the style to stun their opponents, winning 28-26 25-20 25-20.
"This is the team that I was looking for and missed in the previous five matches," Italy coach Mauro Berruto, whose side were beaten by Poland and Bulgaria in the qualifying round before scraping through in fourth spot, told reporters.
It was a bitter disappointment for American captain Clayton Stanley, top points scorer in his side's gold run in Beijing.
"They turned it up a notch and we turned it down," he told reporters. "We worked four years for this, it's kinda tough."
Three-times world champions Italy are looking for their first Olympic title, while Brazil are aiming to recapture the crown they won in Athens, beating the Italians, before losing to the US in the final in Beijing four years ago.
They were clinical against Argentina, their fierce continental sporting rivals.
The impressive spike Murilo Endres fired the winning point for Brazil to the delight of their colourful fans.
"It was our best match so far. Our determination and mindset were crucial," livewire libero Sergio Santos said.
"It's never easy with Argentina but today we were able to attack more efficiently than them."
Russia will take some stopping, judging by their flattening of Poland. With a 3-0 win over Brazil already under their belts in group play, coach Vladimir Alekno's side are oozing confidence. However, he was still not totally satisfied.
"The quarter-final is always the most difficult game. Either you continue, or you are knocked out from the competition. There was a certain psychological pressure on my players and I cannot say that we totally coped with it," he said.