A week before going for a third-straight US Open title, there was nothing wrong with Koepka's long game but he struggled to gauge the speed of the Hamilton Golf and Country Club greens, which were slower than he expected.

American Bradley, at home on a course with lush rough, shot 63 to head Irishman Shane Lowry, South Korean Im Sung-jae, Canadian Nick Taylor, South African Erik van Rooyen and American Roberto Castro by one stroke.

Heavy rain on Wednesday left the short 6,967-yard course soft and allowed players to fire aggressively at the pins.

Rory McIlroy bounced back from a missed cut last week, shooting 67, while defending champion Dustin Johnson battled to a 71. It could have been worse for Johnson, though, as he was three-over par after nine holes.

Among the Australians, Rod Pampling was tied for 59th with a 69, Curtis Luck was 105th with a 71 and Cameron Davis was 135th with a 73.

Koepka had everything dialled in apart from his putter during the opening round of the Canadian Open. PHOTO: Vaughn Ridley/Getty Images.

Koepka, McIlroy and Johnson are among the big names playing in Hamilton with an eye towards peaking at Pebble Beach next week.

Despite his moderate score, Koepka, in his first start since winning the US PGA Championship three weeks ago, was happy with everything except his putting – and a wayward drive into a penalty area at the 10th.

"I liked the way I was hitting the putts, liked the way I was driving the ball, iron play was really good," Koepka told Golf Channel.

"I gave myself some good chances and didn't capitalise."

Bradley, meanwhile, piled up seven birdies to set a cracking early pace.

"It was just solid top to bottom," said the World No.37.

"I'm pretty dialled in right now. Hit almost every fairway, almost every green and then holed some putts. With me it's all about the putter. When that thing gets going I can do stuff like I did today."