Robert Garrigus had a one-shot lead in the opening round of the RBC Canadian Open before play was suspended for the day due to dangerous weather.
The American, who had completed his round before thunderstorms interrupted play, carded a blemish-free nine-under 63 - including five straight birdies in his first six holes.
He added four more on the back nine, helped by a stroke of luck on the 13th when his ball hit a rock to save it from the water.
Closest rival Adam Schenk shot 10 birdies and two bogeys to finish on 62, while compatriot Chris Stroud was third with 65.
Ian Poulter, Hudson Swafford and Byeong Hun An all finished a shot further back, while Kevin Tway was also sitting on six under with three holes left to play.
Two-time defending champion Jhonattan Vegas was two under through 16 holes.
Play was suspended due to the weather at 4.30pm ET (9.30pm Irish time) and resumed just over two hours later. It was stopped again for the day at just after 8pm ET (1am Irish time).
Following the first break, Tommy Fleetwood shot up the leaderboard after hitting two eagles and two birdies in four holes. He finished the day on five under with two holes still to play.
The round is due to resume at 7am ET (12pm Irish time) on Friday, with round two scheduled to get under way half an hour later.