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Bellucci's triumph means the outcome will be decided by the final singles match to come later between American Sam Querrey and Brazil's Thiago Alves.
Novak Djokovic's Serbia awaits the US-Brazil winner in April's quarter-finals.
It took six break-point chances in the eighth game of the final set, but 36th-ranked Bellucci finally cracked Isner when the American netted a slice backhand. Bellucci then held at love, his 21st held service game in a row.
"I think he was a little bit tired in the end of the last set," Bellucci said. "I was just trying to fight because he was making mistakes.
"For me it's very special. Isner is a great player. Of course he was favored. It's always special to beat him. I'm very happy. He has a big serve. It's not easy to play against him on this type of (hardcourt) surface."
Isner, ranked 16th, suffered his fifth loss in a row in five-set matches and left the United States in a live fifth rubber for the first time in 13 years trying to avoid squandering a 2-0 tie lead for the first time since 1994.
"Today was extremely disappointing for me," Isner said. "You can't sugarcoat it. My five-set record is atrocious. It falls on me 100 percent. I've got to get better. I feel bad I didn't come through for my team."
Querrey and Isner won singles matches on Friday to put the Americans on the brink of advancing but Marcelo Melo and Bruno Soares upset the World No. 1 doubles duo of Bob and Mike Bryan on Saturday to keep the Brazilians alive.
The US squad has won a record 32 Davis Cup titles but taken the trophy only once in the past 17 years. The Brazilians are in the World Group for the first time in a decade.
Bellucci forced a fifth set with a break in the last game of the fourth, Isner denying the Brazilian with two service winners before netting a forehand to drop the fourth set.
"My confidence comes and goes, and goes very quickly for me it seems," Isner said. "A lot of that out there today was between the ears.
"For the most part I consider myself a mentally strong player but I wasn't able to free myself up to go for the right shots at the big times.
"I didn't come to play at the crucial moments in those fourth and fifth sets. I started off great. I wasn't able to maintain that. It's on me. I've got to get better."
Isner said he was not bothered by a recent knee injury which kept him out of the Australian Open and praised US captain Jim Courier for giving him wise advice.
"I was listening. I just wasn't able to get it done," Isner said. "This can be a cruel sport when things don't go your way. I would like a chance to redeem myself in a situation like that."