The 25-year-old Pieters rolled in five birdies on the day including his long putt on the par-four, 494-yard No. 9 as he leads...
The 25-year-old Pieters rolled in five birdies on the day including his long putt on the par-four, 494-yard No. 9 as he leads Russell Knox by one shot.
Reigning British Open champion Jordan Spieth and Northern Ireland's Rory McIlroy, who split with his long-time caddie this week, are part of a group of six that shot 67s and are two shots back in Akron, Ohio.
Kevin Kisner, Bubba Watson, Britain's Ross Fisher and Spain's Jon Rahm also shot 67s.
Dustin Johnson, Adam Scott and Adam Hadwin are among a half dozen golfers at 68.
Pieters is the highest-ranked Belgian golfer ever at world number 34. He is playing on the PGA Tour as a Special Temporary Member this season.
He fashioned together a crisp bogey-free round at the Firestone Country Club course.
"I only had two putts from above the hole, the rest of them were pretty straight-forward putts," said Pieters, who hit four of 14 fairways.
"I managed to find a lot of greens today. That was important as well. It was kind of a stress-free round on my back nine."
Knox made bogey on his final hole, spoiling his chance to tie Pieters for the lead.
Spieth is coming off wins at the Travelers Championship and the British Open.
"I really scrambled nicely and made a couple nice long putts in the middle of the round," said Spieth, who finished with five birdies and two bogeys. "When my pace starts to feel good and I start to get dialed in, that is when I know I am hitting it good."
McIlroy is competing in his first tournament since sacking his caddie for the past nine years, J.P. Fitzgerald.
McIlroy, who has struggled this season, is hoping the caddie change produces results and so far the early returns are good. Harry Diamond served as McIlroy's caddie on Thursday.
"It was good. Little weird at the start," said McIlroy, who had five birdies and two bogeys. "It's been nine years since I had someone different on my bag."
"I'm taking a little bit more responsibility on myself and there were a couple of times where I probably should have hit another club, but that's on me, not anyone else."
"I'd much rather be frustrated at my own decision than someone else's."
Phil Mickelson and Australian Jason Day both shot 71 and are tied for 33rd.