"It was kind of a bummer to end the round that way after having a chance (at 59). But overall a great, solid day," Simpson told reporters.
Simpson's bogey-free round, which included seven birdies and an eagle at the par-five 12th where he holed an 18-foot chip shot, left him one shot clear of South Korean Kim Whee and two clear of Chilean teen Joaquin Niemann.
The former U.S. Open winner blitzed the first seven holes on the back nine as he racked up four birdies and an eagle but then missed the fairway at 17 and left a birdie putt at the 18th about a foot short.
"I've worked on a couple things my last week off and seeing some good results from the work," Simpson added.
Jim Furyk broke golf's magic number when he shot a record 12-under 58 at the Travelers Championship in 2016, becoming the first player on the tour to break 60 twice.
Al Geiberger in 1977 was the first to post 59 on the PGA Tour, followed by Chip Beck (1991), David Duval (1999), Paul Goydos (2010), Stuart Appleby (2010), Furyk (2013), Justin Thomas (2017) and Adam Hadwin (2017).
BUBBA FADES
Kim, still seeking his first win since joining the PGA Tour in 2015, mixed 10 birdies with two bogeys during a round where play was suspended for nearly two hours due to lightning in the area.
Niemann, 19, set the early pace after he started hot with five birdies over his opening eight holes and then bookended his back nine with another pair of birdies for a seven-under-par 63.
"I made a couple good birdies in the start of the round," said Niemann, who is the reigning Latin America Amateur champion and former amateur number one.
"After that, I thought I was going really low. My game was feeling really good and my putter was good. So all my game was good today."
Phil Mickelson (66), making his first start since the U.S. Open where he infamously ran down his bogey putt and struck the ball while it was still moving, was in a share of eighth place with defending champion Xander Schauffele.
Bubba Watson, who at number 12 is the highest-ranked player in the field, was in a distant share of 44th after a two-under-par 68.
Watson, who 11 days ago collected his PGA Tour-leading third win of the season, was cruising along at four under until a double-bogey on his penultimate hole, the par-three eighth.
(Reporting by Frank Pingue in Toronto; Additional reporting by Jahmal Corner in Los Angeles; Editing by John O'Brien)
