The native of Blainville, Que., started slowly against McHale, falling behind 3-1 and appearing less sharp than her New Jersey rival.
Several passing shots sailed wide while Wozniak dumped others into the net, including one that lost her the third game, drawing groans from the crowd.
But the world No. 55 stayed aggressive and kept challenging McHale at the net, breaking the 20-year-old in the sixth and tenth games to go up 6-4.
By then, McHale had started to make unforced errors, and Wozniak won the 11th game to take a 6-5 lead.
But McHale's strong serve kept her in the match, and she won the 12th game to tie the set up at six apiece.
In the tiebreaker, Wozniak showed steely resolve and strength. She broke McHale twice, including a brilliant backhand volley at the net to go up 5-3.
Both women held serve until the 12th point, when an errant McHale return gave Wozniak the set.
With fans chanting her name and clapping rhythmically throughout, Wozniak grew stronger in the second set, breaking McHale in the third game on the strength of two blazing cross-court winners that left her American opponent frozen.
The second of those was a game-winner, eliciting roars and shouts of approval from the 9,000 fans in attendance.
McHale got her energy back and the speed of the groundstrokes picked up, which began to cause Wozniak problems.
The American, also displayed a strong, if erratic, serve throughout, recorded five aces to three double faults. But one of those aces gave her the fifth game to pull to within 3-2.
Wozniak blasted an ace in the sixth game to hold serve and go up four games to two, and she outlasted McHale in the following game to break the American to set up a potentially-decisive game.
But the rain grew steadier just as Wozniak was set to serve for the match, and chair umpire Mariana Alves called a rain delay that lasted for more than six hours before tournament organizers cancelled the evening session.
The Canadian had upended former world No. 1 Jelena Jankovic 6-2, 6-3 Thursday to advance to the third round against McHale.
If she wins her third-round match, she'll move on to face the winner between No. 7 seed Caroline Wozniacki of Denmark and Varvara Lepchenko of the United States.
Earlier on Friday, Czech mates Petra Kvitova and Lucie Safarova were the first to advance to the quarterfinals.
Kvitova, the world No. 6 player, easily handled France's Marion Bartoli, 6-1, 6-1, at centre court.
The 22-year-old advances to her first-ever Rogers Cup quarterfinal after a strong season in which she reached the semifinals of the Australian Open and the French Open.
She'll face the winner of the match between unseeded players Tamira Paszek of Austria and Carla Suarez Navarro of Spain.
Meanwhile, at National Bank court, the 23rd-ranked Safarova needed more than two hours to upend world No. 5 Samantha Stosur of Australia, 7-6 (9), 7-6 (5).
The top remaining seed, world No. 2 Agnieszka Radwanska of Poland, saw rain wash out her showdown against unseeded South African Chanelle Scheepers for a berth in the quarterfinals.
Radwanska's match, along with the Li Na vs. Sara Errani third-round showdown, were put off until Saturday.
In doubles action, top-seeded American duo Liezel Huber and Lisa Raymond were the first pair to advance to the semifinals. They moved on after Sabine Lisicki of Germany and Peng Shuai of China retired.
Other doubles winners were No. 3 seeds Nadia Petrova of Russia and Katarina Srebotnik.
Also advancing in doubles were the unseeded pair of American Bethanie Mattek-Sands and India native Sania Mirza.
NOTES: Wozniak has not advanced further than the second round in eight previous Rogers Cup appearances ... Faye Urban of Windsor, Ont., was the last Canadian woman to win the Rogers Cup. She did it in 1969, defeating fellow Canadian Vicky Berner in the final of what was then called the Canadian Open.