Errani and Vinci, the top-ranked doubles team, won 6-2, 6-2 against an understrength American team. The U.S. was without Serena and Venus Williams and Sloane Stephens because of injuries.
"I'm really proud of the team, they played their hearts out every single match," U.S. captain Mary Joe Fernandez said. "We knew it was going to be an uphill battle."
The U.S. led 2-1 after Lepchenko beat Errani 7-5, 6-2 in Sunday's first reverse singles. But Vinci stopped Jamie Hampton 6-2, 4-6, 6-1 to set up the crucial doubles for Errani and Vinci, who ended the best-of-five World Group series.
"They both played a great match and Roberta did a couple of really exceptional shots at the net," Italy captain Corrado Barazzutti said. "I don't really say much to them when they play, they're the best, what can you say? I try not to bother them when they play, they know each other so well."
Italy will host defending champion Czech Republic in the semifinals, a rematch of last year's semis. The Czech Republic defeated visiting Australia 4-0.
In the other semifinal, it's Slovakia at Russia. Slovakia topped host Serbia 3-2, and host Russia downed Japan 3-2.
Italy was a big favorite to earn a third straight victory over the U.S. after beating the Americans in the Fed Cup finals in 2009 and '10.
Excellent play from Lepchenko, however, put the U.S. ahead after an error-strewn performance from Errani.
Lepchenko raced ahead 4-2 before Errani rallied to lead 5-4 and was serving for the set. But the 21st-ranked American broke again and took the next two games, securing the opening set when Errani hit a shot into the net.
Lepchenko moved in front 4-1 in the second set to earn a second straight victory in her Fed Cup debut after beating Vinci in three sets on Saturday.
"It was a long match yesterday," Lepchenko said. "So I was up and down a bit, but once my body warmed up I started playing better and better, and then in the second set I got pumped and really got going. Clay is one of my favorite surfaces. I was born on clay, so I guess it comes to me naturally."
Vinci then kept Italy in the series, despite playing with a thigh injury from Saturday's loss. After inconsistent play in the first two sets, Vinci pulled ahead 5-0 in the third.
"It was an important third set that Jamie lost," Fernandez said. "We could have capitalized there but we didn't. But this is the closest we've come against the Italian team, and we were playing away.
"I'm really proud of everyone, how they handled the atmosphere. ... There were a lot of new experiences and overall I think it's a positive experience."
Vinci played even better in doubles and was particularly sharp at the net as she improved her record to 18-0 in Fed Cup doubles— the best in the competition's history.
"It's maybe the best day of my career," Vinci said. "We really wanted it. We were very tired but today we managed to show a lot of heart, a lot of personality. We are a great team. I had to have various treatments and physiotherapy, massages almost until midnight. Yesterday was a horrible day and I'm glad I managed to come back today. "
Errani and Vinci won the doubles title at the Australian Open last month, their third victory in the last four Grand Slams, and they were in command against Lepchenko and Huber.
"Of course, it's very disappointing today not to win, disappointing to lose again to them, that's what hurts the most," Huber said. "They (Vinci and Errani) are the weapon. They're like sisters and best friends. They can read each other's body language. That's why they're No. 1 and think they'll be that for many years to come. They both have different styles and they stick to that. I really enjoy watching them play, but not against me."
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