Pospisil sent Canada to the quarterfinals after Japan tied it at 2 with fourth-ranked Kei Nishikori's 3-6, 6-3, 6-4, 2-6, 6-4 victory over Milos Raonic in the first match Sunday.
Pospisil, ranked 62nd in the world, ended the match with an ace.
Canada will face Belgium in quarterfinals. Belgium beat visiting Switzerland 3-2.
On Saturday in doubles, Pospisil and Daniel Nestor beat Soeda and Yasutaka Uchiyama 7-5, 2-6, 6-3, 3-6, 6-3. The teams split the opening singles matches Friday. Raonic topped Tatsuma Ito 6-2, 6-1, 6-2, and Nishikori beat Pospisil 6-4, 7-6 (5), 6-3.