The big-serving Raonic banged down 18 aces in a 6-3 7-5 defeat of lowly-ranked Finn Patrick Kaukovalta to put Canada 1-0 ahead in the best-of-three tie.
Raonic began the match with a game of aces and ended it with another one after little more than an hour.
Both teams were hit by injuries with Canada's top-ranked player Felix Auger-Aliassime unable to take to court while Finland's number one Emil Ruusuvuori was also ruled out.
Former world number three and Wimbledon runner-up Raonic has played only a handful of matches this year but had too much firepower for 782nd-ranked Kaukovalta who put up a good fight in his first ever Davis Cup singles rubber.
Gabriel Diallo will send Canada into the semi-finals if he beats Otto Virtanen in the second singles rubber.
Canada are looking to retain the title they won against Australia last year in Malaga and also make it a sweep for the country after the women won the Billie Jean King Cup.
