OAKVILLE, July 27, 2013 (AFP) - Mahan's eight birdies were enough to seize the lead from early second-round pace-setter Merrick, who equalled the Glen Abbey course record with a 10-under 62 that included an eagle and eight birdies.
Mahan had a 13-under-par total of 131, with Merrick on 11-under 133. Former Masters champion Bubba Watson was alone in third place after a 67 for 135, and it was a further stroke back to Patrick Reed (68) and Australian Aaron Baddeley (68) on 136.
Mahan, who played in the final group on Sunday at the British Open and finished ninth in the year's third major, birdied his last three holes to get past Merrick.
Mahan had got his round going with a similar burst of three birdies at the second third and fourth, and also birdied 11 and 13.
Merrick's round matched the course record set by Australian Greg Norman in 1986, but he acknowledged such a feat wasn't looking likely after a poor warm-up on Friday morning.
"I wasn't hitting it that great on the range and I didn't feel that good," Merrick said. "Maybe that makes you focus a little bit more, try to figure out what's going on, and I was able to kind of work a couple things out."
Merrick, who claimed his first US PGA Tour title with a playoff victory at the Northern Trust Open in Los Angeles in February, jump started his round with an eagle at the par-five second hole. He added back-to-back birdies at the seventh and eighth, and his six birdies on the back nine included another brace at 17 and 18.
"It was a great day," said Merrick, who hit 17 of 18 greens in regulation. "I got off to a good start, hit it over the green on two and chipped in for eagle.
"My game has been feeling pretty good lately. Everything kind of clicked today."
Hopes of a first Canadian victory in 59 years in the country's national open were looking dim after 36 holes. Mike Weir, who has been hampered by various injuries since winning the Masters in 2003, was the top Canadian after a five-under 67 that left him tied for 26th on 140.
Leading second-round scores on Friday in the US PGA Tour $5.6 million Canadian Open (USA unless noted, a-denotes amateur, par-72)
131 - Hunter Mahan 67-64
133 - John Merrick 71-62
135 - Bubba Watson 68-67
136 - Patrick Reed 68-68, Aaron Baddeley (AUS) 68-68
137 - Tommy Gainey 73-64, Chris Kirk 68-69, James Hahn 69-68
138 - Jason Bohn 70-68, Charley Hoffman 69-69, James Driscoll 69-69, Greg Owen (ENG) 70-68, David Lingmerth (SWE) 67-71, Scott Brown 66-72, Hideki Matsuyama (JPN) 69-69
139 - Andres Romero (ARG) 69-70, Cameron Tringale 72-67, Roberto Castro 69-70, Jim Furyk 72-67, Mark Wilson 70-69, Camilo Villegas (COL) 74-65, Alistair Presnell (AUS) 72-67, Jeff Maggert 72-67, Brandt Snedeker 70-69, Kyle Stanley 68-71
140 - William McGirt 71-69, Casey Wittenberg 71-69, J.J. Henry 73-67, Mike Weir (CAN) 73-67, Jason Kokrak 72-68, Ryan Palmer 70-70, David Mathis 71-69, Fabian Gomez (ARG) 72-68, Matt Kuchar 66-74, Billy Horschel 71-69, Brendan Steele 65-75, Scott Verplank 72-68, Rory Sabbatini (RSA) 69-71, Tim Petrovic 71-69, Morgan Hoffmann 70-70, Scott Gardiner (AUS) 66-74