ReutersPiercy mixed two eagles and five birdies with a lone bogey at a rain-softened Hamilton Golf & Country Club for an eight-under-par 62 that set the course record and gave him a one-shot lead over compatriot William McGirt and Britain's Greg Owen.
American Robert Garrigus (64) was a further shot back while 2011 Masters champion Charl Schwartzel and former world number one Vijay Singh were among a pack of seven golfers sitting three shots back of the leader.
Owen, who started on the back nine, opened with a disastrous double-bogey at the par-four 10th and a bogey at the par-four 11th but was red-hot the rest of the way for his seven-under 63.
"I am putting well and when I putt well I feel confident," said Owen. "I don't always putt well, it's a poor part of my game, so when I start feeling it a little bit it just gives me more confidence to go out and hit better shots."
Els, fresh off his Open triumph on Sunday, had a steady start but finished with a two-over-par 72 after failing to recover fully from a triple-bogey at the par-four 11th hole.
World number nine Matt Kuchar, the highest ranked player in the $5.2 million event, fired a three-under 67 while defending champion Sean O'Hair stumbled to a five-over-par 75.
Jim Furyk, who won his first of two consecutive Canadian Open titles when it was last held at the tree-lined course in Hamilton, Ontario in 2006, was five shots off the pace after offsetting four birdies with four bogeys for an even-par 70.
Adam Hadwin kept alive hopes for the tournament's first Canadian winner since 1954 when he turned in a four-under 66 that left him four shots off the lead.
