After coming into Sunday's match with a 2-1 advantage, Raonic sealed Canada's best-of-five series win with a 6-4, 6-4, 3-6, 7-5 decision over Seppi in the day's first reverse singles match.
The Canadians had grabbed the lead Saturday when the doubles team of Vasek Pospisil and Daniel Nestor dispatched the Italian duo of Daniele Bracciali and Fabio Fognini.
Canada, which prevailed in its first-ever Davis cup meeting with Italy, now goes on to face Serbia, which defeated the United States in another Sunday quarterfinal.
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