He led by one stroke from Scotsman Russell Knox, who bogeyed the last.
"I only had two putts from above the hole. The others were pretty straightforward putts," Pieters said after heading the select 76-player field in his first competitive round on the course.
"I was pretty terrible from the tee, but managed to hit a lot of greens. Kind of a stress-free round."
Spieth, showing no signs of a let-down following his British Open triumph two weeks ago at Royal Birkdale, was two shots off the pace along with four-times major winner McIlroy, playing alongside new caddie Harry Diamond.
The Northern Irishman split with his caddie of nine years, J.P. Fitzgerald, following the British Open.
"It was good. A little weird at the start," McIlroy said about the new partnership with best friend Diamond. "Obviously, it’s been nine years since I’ve had anyone else on my bag."
Short-game master Spieth wielded his magic putter to sink consecutive monster birdies late in his round, before ending his day with a sand save at the last, holing a seven-footer for par.
"I really scrambled nicely, and made a couple of really long putts back-to-back in the middle of the round that put us at three (under) when I felt like I played a one or two-under round," said Spieth, winner of his last two events.
Joining Spieth and McIlroy on 67 were double Masters winner Bubba Watson, Englishman Ross Fisher, Spaniard Jon Rahm and American Kevin Kisner in a round that was played two hours earlier than originally planned to avoid threatening thunderstorms in the forecast.
Another shot away was world number one Dustin Johnson, who bombed a 439-yard drive on his way to a birdie at the downhill par-five 16th hole.
Knox vaulted up the leaderboard boosted by an eagle at the par-five second, his 11th hole, when his 76-yard pitch shot trickled back into the cup.
"I hit a poor drive and just laid up," he said. "I was able to judge the distance right and it kind of rolled up to the top of the hill and trickled back and went in. Obviously, it was a huge bonus to make eagle."
(Reporting by Larry Fine in Safety Harbor, Florida; Editing by Frank Pingue and Andrew Both)
Share
