AKRON, Ohio — Rory McIlroy had his best friend on the bag and his best start on American soil in 18 months. For Jordan Spieth, it was more of the same with two long putts and another comment filled with bravado directed at his caddie.

Both of them opened with a 3-under 67 at the Bridgestone Invitational, leaving them two shots behind Thomas Pieters of Belgium.

Pieters, playing only for the sixth time since he challenged briefly at the Masters this year, holed a 30-foot birdie putt on his final hole at Firestone to finish a day of good scoring with a one-shot lead over Russell Knox.

The opening two rounds were moved up to the morning because of a forecast of thunderstorms in the afternoon.

McIlroy split with J.P. Fitzgerald, his caddie of nine years, after the British Open. He decided to use Harry Diamond, who played amateur golf for Ireland and was the best man at McIlroy’s wedding, for the Bridgestone Invitational and the PGA Championship next week.

It didn’t have much bearing on McIlroy’s game, which was fine. McIlroy won at Firestone the last time he played it in 2014.

“We both did the numbers and I sort of consulted him a couple of times. Yeah, it was good,” McIlroy said. “There was a couple of shots that I hit or a couple of clubs that I pulled that I maybe should have just thought a little bit more about. It’s been a while since I’ve paced yardages off and written notes in my book.”

One of them was at No. 9, his last hole, when he went some 50 feet long on his approach and three-putted for bogey. Even so, it was a solid start, and that’s what has held back McIlroy in recent months when he missed three cuts in four tournaments, and then started poorly at the British Open.

Spieth has no such concerns, having won two straight events going into this World Golf Championship with an eye toward next week at the PGA Championship and his shot at becoming the youngest player to complete the Grand Slam.

Winning a major turned this into a great year for Spieth, regardless of what happens at the PGA Championship. He is feeling as good as ever about his game, particularly the way he finished off Royal Birkdale.