The co-leaders have a one-stroke lead over Swede David Lingmerth (69), with American Brian Stuard (67) two strokes behind in the World Golf Championships event.
"I felt like Mr. Havercamp out of Caddyshack, not knowing where my golf ball was going," Day told CBS television, referring to the comedy golf movie that remains a favourite among tour players who were not even born when the film was released in 1980.
"I just had to do something. Obviously, the goal is to win the tournament. I hit some pretty errant shots out there and the short game saved me.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"There was a section of the round where I was losing everything right and getting myself into trouble in and around the greens.
"There are a lot of negatives about this round, but I guess the positive is I’m still tied for the lead and I haven’t hit the golf ball that great this week."
Day, who has won seven times in the past 12 months, would like to find his form, and not only to add an eighth title. He also has an eye on the British Open at Royal Troon in two weeks.
“I’m pretty exhausted right now. I hope I find something in my sleep,” he said.
Day and the 37-year-old Piercy are the only two players in the field to have posted three straight sub-70 rounds at Firestone.
Piercy, a three-time PGA Tour winner, took plenty of positives from his tie for second at the U.S. Open at Oakmont two weeks ago.
"It definitely helped the confidence,” said Piercy, who hit eight of 14 fairways in his third round.
“Trying to hit those fairways at the Open, and hitting these fairways doesn’t seem quite as hard, but it’s still difficult out here. Knowing I drove it well there and I need to drive it well here has helped my confidence.”
U.S. Open champion Dustin Johnson fired the day’s low round of 66 to move up 17 spots and into a tie for fifth with South African Charl Schwartzel (67) and American William McGirt (70), three strokes off the pace.
(Reporting by Tim Wharnsby in Toronto; Editing by Andrew Both)
