American Robert Garrigus shot a third-round 64 to move into a one-shot lead at the RBC Canadian Open in Ontario.
Garrigus is a shot clear of a his fellow American William McGirt, who had a second successive 66 round the par 70 Hamilton Golf and Country Club layout.
Garrigus' 16-under total of 194 broke the 54-hole scoring record at the Canadian Open that was set more than a half-century ago by Arnold Palmer.
On rain-softened greens, Garrigus used his power and some fine putting to go one shot better than Palmer in 1955 at Weston Golf & Country Club, a total matched two years ago by Dean Wilson at St. George's Golf & Country Club.
Palmer went on to win in 1955 for the first of his 62 PGA Tour titles. Garrigus will be aiming for his second on Sunday.
United States players filled the top 10 places at the end of the third round.
Garrigus was pressed continually by McGirt and Piercy who around the turn joined him in a three-way tie for the lead.
Piercy dropped shots at the 10th and 13th, although he recovered with birdies at the 15th and 17th.
McGirt, too, bogeyed the 13th before taking a birdie at the 16th.
Scott Stallings, Chris Kirk and Bo Van Pelt finished tied for fourth place on 12 under after Stallings and Kirk both shot seven-under par 63s.
Kirk hit seven birdies in eight holes between the third and the 10th in a flawless round and Stallings scored three birdies and an eagle at the 10th before dropping his only shot three holes later.
Van Pelt made a bogey at the first but recovered with four birdies in his round and no other dropped shots.
South Africa's Retief Goosen matched Stallings' and Kirk's best rounds of the day with a 63 that included seven birdies and no dropped shots to take him 42 places up the leaderboard into a tie for 11th.