The red-hot finish left Piercy, who is looking to snap a 14-month victory drought on the PGA Tour, two shots clear of Japan's Hideki Matsuyama and Canada's Adam Hadwin.
Piercy reached the turn at three-under but suddenly found another gear on the closing stretch and capped his bogey-free round with a birdie at the par-five 18th where he rolled in a six-foot putt.
Matsuyama had four birdies after nine holes and stormed out of the turn with another four over a sizzling six-hole stretch before dropping a shot at his final hole, the par-four ninth, to grab the early clubhouse lead at seven-under.
The Japanese, who last triumphed on the PGA Tour at the 2017 WGC-Bridgestone Invitational, felt confident after his round.
"It was a good round today. Played well. Hopefully I can keep it going tomorrow and the rest of the week," Matsuyama told reporters through a translator. "I was putting very well today and that was the key to the round."
World number one Brooks Koepka, a four-times major champion who has yet to triumph at a regular PGA Tour event, mixed five birdies with a lone bogey for a four-under-par 67 that left him five shots off Piercy.
Among the other notables in the field are Bryson DeChambeau (66), Tony Finau (66), former Masters winner Patrick Reed (69), Australian Jason Day (69) and five-times major winner Phil Mickelson (74).
(Reporting by Frank Pingue in Toronto; Editing by Ian Ransom)
