Krejcikova, the world number 10, put the Czechs ahead with a 6-2, 6-1 singles win over 18-year-old Canadian Marina Stakusic.
However, Wimbledon champion Marketa Vondrousova, having arrived late from the WTA Finals in Cancun, was stunned by the 21-year-old Fernandez who claimed a 6-2, 2-6, 6-3 win.
"I feel like it was a great match, and she played amazing shots. I was a bit unlucky in the third set," Vondrousova said.
The showdown went to the doubles clash, with 11-time winners the Czechs failing to clinch victory, as Krejcikova and Siniakova lost 7-5, 7-6 to Dabrowski and Fernandez.
"Definitely, it is a great day for us, Team Canada. In the singles match I was super happy to have the crowd with me, cheering me on that really help me," said Fernandez.
Earlier, Jasmine Paolini defeated Tamara Zidansek and Martina Trevisan beat Kaja Juvan as Italy secured a 2-0 victory over Slovenia in Seville on Saturday to reach the Billie Jean King Cup final and stay on course for their fifth crown in the team competition.
Paolini survived a wobble midway through the second singles clash before the world number 30 battled past Zidansek 6-2, 4-6, 6-3 to send her team through to the title clash, sparking huge celebrations from Italian fans at the Estadio de la Cartuj.
Paolini was trailing 2-0 in the decider but quickly rediscovered her rhythm to dash the hopes of first-time semi-finalists Slovenia, who were the lowest-ranked of the 12 countries in the competition.
Trevisan earlier put Italy in front in their quest to make the final of the event for a sixth time with a battling 7-6, 6-3 victory over Slovenian world number 104 Juvan.
After edging a tight opening set in the tiebreak, the 43rd-ranked Trevisan fought back from 3-1 down in the second set and grabbed the decisive break in the eighth game to secure the win in two hours and 20 minutes.
"I have to say that today was really tough," Trevisan said as Italy tasted success in their first semi-final appearance in the competition since 2014. "A lot of emotions and a lot of firsts ... A result obtained starting from very far."
Italy's last triumph in the competition came 10 years ago, and they will face stiff competition in Sunday's title clash from Canada who have yet to win the tournament.
