* Spain on brink of exit in Canada
* Djokovic's Serbia lead Belgium 2-0 in Charleroi
* Holders Czech Republic level with Switzerland
* Germany's Kohlschreiber retires hurt v Argentina
LONDON, Feb 1 (Reuters) - Top seeds Spain moved to the brink of a shock Davis Cup exit on Friday when their weakened team slumped to a 2-0 deficit at the hands of Canada in a world group, first round tie played on lightning quick Vancouver hardcourts.
Novak Djokovic was in no mood for upsets, however, and earlier got right back in the groove after his Australian Open triumph with a crushing defeat of Olivier Rochus as Serbia surged into a 2-0 lead in Belgium.
Holders Czech Republic were locked at 1-1 after the first day against a Switzerland side minus world number two Roger Federer - Tomas Berdych levelling the tie after Lukas Rosol had been outplayed by Stanislas Wawrinka.
Germany's hopes of beating Argentina in Buenos Aires suffered a cruel blow, however, as Philipp Kohlschreiber was forced to retire with a hamstring injury when serving for the match at 5-4 in the fifth set against Carlos Berlocq.
A later win for Juan Monaco over Florian Mayer meant Argentina ended the day 2-0 ahead as did France and Kazakhstan in their ties against Israel and Austria respectively.
Croatia and Italy were locked at 1-1 in Turin while the United States took a commanding 2-0 lead over Brazil in Jacksonville after Sam Querrey dispatched Thomaz Bellucci in straight sets and John Isner thumped Thiago Alves.
Spain, five-times winners of the Davis Cup, were weakened by the absence of top 15 players Rafa Nadal, David Ferrer and Nicolas Almagro and the Canadians, who have never won a world group tie, pounced to take advantage.
Big-serving Milos Raonic was expected to bludgeon his way past Alberto Ramos and duly delivered when he sealed a 6-7 6-4 6-4 6-4 victory over the world number 51 with two thumping aces.
Frank Dancevic, ranked 161, provided the real shock of the day, though, humbling world number 34 Marcel Granollers 6-1 6-2 6-2 to give his country a 2-0 lead going into Saturday's doubles.
"I think I played the match of my life today," said the 28-year-old journeyman.
"I just went in with a lot confidence, like just had to go all or nothing today. Anytime I had an opportunity I went for it.
"This isn't over yet. We've (got) to go strong, these guys aren't finished yet, they're going to come strong tomorrow.
"They've got a great doubles team ... we just have to go hard until the last ball this weekend."
Less than a week after claiming a third consecutive Australian Open title by beating Andy Murray in Melbourne, world number one Djokovic quickly adapted to the indoor clay court at the Spiroudome in Charleroi to blow away Rochus 6-3 6-2 6-2.
Serbia's Viktor Troicki, playing in place of the absent Janko Tipsarevic, battled back from two sets down to beat David Goffin in the opening rubber.
Troicki, who won the deciding rubber when Serbia beat France to win the Davis Cup for the first time in 2010, produced a gutsy display after being outplayed for two sets.
Friday's match began to spin away from Goffin in the third set as Troicki clawed his way back to win 1-6 3-6 7-6 6-4 6-4.
On the eve of the match, Serbia had criticised the playing surface, saying the decision to lay a clay court on top of a wooden floor was dangerous.
The International Tennis Federation (ITF) issued a statement saying they were happy with the court and it seemed fine for Djokovic who fired down 20 aces in a one hour 39 minute breeze.
"The serve was incredible and I think that helped me to get the match done in straight sets which was important," Djokovic said. "It wasn't easy playing on clay after a long time indoors and coming from Australia quite late.
"At the start I had trouble with movement and getting used to the conditions but I played my best tennis when I needed to."
A strong France lineup had too much for visitors Israel on the opening day in Rouen where Jo-Wilfried Tsonga needed four sets to beat Amir Weintraub before Richard Gasquet made short work of Dudi Sela.
"It is the perfect scenario because we've got two wins under our belt," team captain Arnaud Clement told reporters. "We will need to be very focused so we can wrap this up tomorrow." (Reporting by Martyn Herman; Editing by Ken Ferris)