The 48-year-old finished five-under par on 67 in a calm performance, amid patches of heavy rain, at Glen Abbey on Thursday.

Meanwhile, Garrigus carded five straight birdies from the 2nd hole on the way to an error free nine-under 63.

As good as the American's round was, the 40-year-old says it could have been better, even historic, believing that with a bit more luck he could have flirted with a 59.

Instead Garrigus's brilliant round added up to no more than a one-shot clubhouse lead over Adam Schenk, who birdied his closing three holes, while fellow American Chris Stroud lurked one shot further back by finishing with four straight birdies.

"Could have been 59 watch there if a couple putts didn't lip out," said Garrigus, who has just one top-10 result this season and missed the cut in 11 of 24 starts.

"Lipped out on 10, I had a lip out on 17. Got lucky on the par-5 13th, hit right into the middle of the water and hit a rock and bounced over the thing and I made a birdie."

Garrigus reacts to his tee shot on the 17th tee during the first round at the RBC Canadian Open at Glen Abbey Golf Club. PHOTO: Minas Panagiotakis/Getty Images.

World No.1 Dustin Johnson, looking to bounce back after his first missed cut this year at The Open Championship, shot a rollercoaster 68 that included an eagle, six birdies and four bogeys to leave the big-hitter five off the early pace.

"There was some really good stuff and some not so good stuff," said the American, who has already picked up two wins this season.

"All-in-all it was a pretty good day just a few loose shots. Every time I was out of position I made a bogey but I made a bunch of birdies and an eagle."

With the weather forecast predicting thunderstorms later on Thursday, officials were scrambling to complete the opening round before the severe weather returns.