Even without top players Jannik Sinner and Lorenzo Musetti, the Italians were way too strong in front of a partisan crowd with singles wins for Matteo Berrettini and Flavio Cobolli.
Former Wimbledon runner-up Berrettini was first up in the Supertennis Arena against Jurij Rodionov and after running into trouble in the second set, he won 6-3 7-6(4).
An inspired Cobolli, Italy's number one for the week, then thrashed Filip Misolic 6-1 6-3 to complete the job.
"We all want to win this cup and I felt the support of the bench, it was unbelievable," Cobolli said. "When I play for them, it is something which I cannot describe."
Berrettini won all six of his Davis Cup matches for Italy in 2024 as they defended their title and captain Filippo Volandri kept faith with the big server on Wednesday.
It was comfortable enough for Berrettini in the first set which he sealed with an ace.
A fault with the lighting inside the arena caused a stoppage in the second set and when play resumed it was Rodionov who clicked into gear first to break serve for the first time.
The 177th-ranked Rodionov served for the second set at 5-3 but some untidy errors allowed Berrettini to break back.
Berrettini then went 0-40 down in the next game but he served his way out of trouble to get back on level terms.
A double-fault by Rodionov handed Berrettini the initiative in the tiebreak and the Italian brought up a match point with a reflex volley before a big serve finished it.
While the first match was a contest, the second was a procession as Cobolli needed just over an hour to send Italy into the semi-finals on Friday.
Belgium, seeking to win the competition for the first time, booked their place in the semi-finals with a 2-0 victory over France on Tuesday.
Four-time grand slam champion Jannik Sinner, who led Italy to titles in 2023 and 2024, opted to sit out the Davis Cup final this season.
The Davis Cup Final 8 in Bologna is the sixth edition of the revamped event that has the champion decided at a neutral site.
No. 1-ranked Carlos Alcaraz, the six-time Grand Slam champion who is coming off a runner-up finish to Sinner at the ATP Finals in Turin on Sunday, withdrew with a hamstring injury.
Spain will take on the No. 4-seeded Czech Republic, and No. 2 Germany will play Argentina in the last two quarter-finals on Thursday.
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