Kvitova, who also won the WTA Championships last October, is without a title this year but her 6-1 6-1 thrashing of ninth-seed Marion Bartoli showed she will be difficult to beat.
Bartoli said Kvitova was almost impossible to defeat when she played at her peak and rated her a harder opponent than either 14-times grand slam winner Serena Williams or world number two Maria Sharapova when in form.
"She takes away from you the control of the match," Bartoli said. "I never felt like this against Serena or Sharapova. When I was serving well, she returned a missile."
Aiding the cause of Kvitova, who will play Tamira Paszek in the quarter-finals of the $2 million tournament, was Friday's withdrawal of Azarenka with a knee injury.
Azarenka, who would lose her top ranking should Poland's Agnieszka Radwanska win here, also withdrew from the next week's Cincinnati Open next week but hopes to be fit for the August 27-September 9 US Open, the year's final Major.
"I hope it's a short-term thing. I have to just stop and take a little bit of time off because I had absolutely no rest for a long period of time," she said.
"If I cannot play here, there is no chance for me to play anywhere else. My first goal is to get healthy and make sure I'm 100 per cent ready to play the US Open."
The Belarusian's withdrawal came two days after Sharapova pulled out due to a stomach virus that will also prevent her from playing in Cincinnati.
Also assisting the Montreal hopes of Kvitova and Radwanska, whose Friday match against Chanelle Scheepers is in doubt due to rain that has again postponed play, is the exit of last year's finalist, Australian Sam Stosur.
US Open champion Stosur lost her third-round clash with 16th seed Lucie Safarova 7-6 7-6 after wasting several chances.