Thomas Pieters holds the first-round lead at the WGC-Bridgestone Invitational after carding a 5-under-par 65 on Thursday at Akron, Ohio.
Pieters recorded five birdies during the bogey-free round for a one-shot lead over Russell Knox. The Belgian finished his day in style by sinking a 31-foot birdie putt on his final hole, the par-4, 494-yard No. 9 at Firestone Country Club.
The 25-year-old Pieters is playing as a Special Temporary Member on the PGA Tour. He is ranked 34th in the world golf rankings, making him the highest-ranked Belgian ever.
He only hit four of 14 fairways but was able to avoid mistakes during his crisp round.
"I only had two putts from above the hole, the rest of them were pretty straight-forward putts," Pieters said afterward in a post-round television interview. "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 had an opportunity to tie Pieters but bogeyed his final hole. Knox eagled the par-5, 526-yard No. 2 and also posted four birdies but had two bogeys.
Spieth, who is coming off wins at the Travelers Championship and The (British) Open, had five birdies and two bogeys. He said he overcame some putting issues during his round.
"I really scrambled nicely and make a couple nice long putts in the middle of the round," Spieth said afterward. "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 also registered five birdies and two bogeys in his first tournament since separating from J.P. Fitzgerald, his caddie of nine years.
McIlroy said he split with Fitzgerald because "in order to preserve a personal relationship, I had to sacrifice a professional one."
Boyhood friend Harry Diamond served as McIlroy's caddie on Thursday.
"It was good. Little weird at the start," McIlroy said. "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."