Missing top players Rafael Nadal, David Ferrer, Nicolas Almagro and Fernando Verdasco, the champions in three of the last five years, and runners-up in 2012, were blown off court in Vancouver in the first round clash.
Canadian veteran Frank Dancevic, ranked 166th in the world, stormed to a 6-1, 6-2, 6-2 victory over Marcel Granollers, ranked 34th, to put the hosts up 2-0.
World number 15 Milos Raonic had opened the tie with a 6-7 (5/7), 6-4, 6-4, 6-4 win over Albert Ramos.
Spain, who haven't lost in the first round since 2006, now need Granollers and Marc Lopez to beat Daniel Nestor and Vasek Pospisil in the doubles later Saturday to stay alive in the tie.
"It was beautiful tennis – unfortunately it was painful for us," admitted Spain captain Alex Corretja. "We know it may be a miracle if we want to win this tie."
Canada captain Martin Laurendeau refused to accept that Spain were handicapped by the absence of their marquee names.
No easy playing
"Spain is No.1 in the world, I think Spain could field three Davis Cup teams," he told www.daviscup.com.
On Friday, world number one Novak Djokovic steered Serbia to the brink of the quarter-finals, easing past Belgium's Olivier Rochus on a court he had described as the worst he's ever encountered.
Australian Open champion Djokovic needed just an hour and 39 minutes to beat 32-year-old Rochus 6-3, 6-2, 6-2 to give Serbia a 2-0 lead after Viktor Troicki took five sets to defeat David Goffin.
"It wasn't easy playing on clay after a long time indoors and coming from Australia quite late," said Djokovic.
"At the start I had trouble with movement and getting used to the conditions but I managed to play my best tennis when I needed to."
Troicki battled back from two sets down against Goffin to win 1-6, 3-6, 7-6 (7/5), 6-4, 6-4 as Serbia, the 2010 champions, eyed reaching the quarter-finals for a fourth consecutive year.
Sam Querrey and John Isner powered 32-time champions United States to a 2-0 lead over Brazil in Jacksonville, Florida.
Querrey fired 15 aces to beat Thomaz Bellucci 6-3, 6-4, 6-4 before Isner, who missed the Australian Open with a knee injury, pounded 32 aces past Thiago Alves in a 6-3, 7-6 (7/4), 6-3 victory.
With their formidable doubles duo of Bob and Mike Bryan due up on Saturday the United States looked to be in prime position to advance to face either Belgium or Serbia.
Marin Cilic gave Croatia the lead over Italy in Turin with a 6-1, 6-7 (6/8), 4-6, 6-3, 6-2 win over Paolo Lorenzi.
But Andreas Seppi pulled the hosts level with a 6-2, 6-7 (2-7), 6-4, 6-4 win over Ivan Dodig.
The winners of that tie will meet either Canada or Spain.
France took a commanding 2-0 lead over Israel in Rouen as Arnaud Clement made his debut as national coach.
Jo-Wilfried Tsonga, the world number eight, beat Amir Weintraub 6-3, 6-3, 4-6, 7-5 before world number 10 Richard Gasquet breezed past Dudi Sela 6-3, 6-2, 6-2.
Defending champions Czech Republic finished the day in Geneva at 1-1 against Switzerland.
Stanislas Wawrinka, the Swiss team's number one in the absence of Roger Federer, saw off Lukas Rosol, the shock conqueror of Nadal at Wimbledon last year, 6-4, 6-3, 6-4.
World number six Tomas Berdych then defeated Finland-born Henri Laaksonen 6-3, 6-2, 6-7 (5/7), 6-1.
The winners of that tie will meet either Kazakhstan or Austria in the last eight.
Kazakhstan led 2-0 after the opening day in Astana while Argentina took a 2-0 lead over Germany in Buenos Aires. – Sapa-AFP