Hideki Matsuyama, of Japan, fired a five-under 67 to join Canadian David Hearn and Americans Kevin Na and Charley Hoffman in the lead at TPC Sawgrass, one shot ahead of six Americans.
Northern Irishman McIlroy, who played with Spieth and world number seven Jason Day, put together a steady three-under 69.
Australian Day matched 26-year-old McIlroy, but Masters champion Spieth struggled to a three-over 75 and finished the day equal 109th, eight shots off the pace.
Matsuyama missed several fairways early but recovered to notch six birdies and just a lone bogey at the first, his 10th hole.
“I putted very well today. That was the difference,” the 23-year-old told reporters through an interpreter.
“The key to this golf course for me anyway is to keep the tee shots in the fairway, then score well on the back nine.”
Last year's Memorial tournament winner talked down his place among the budding rivalry of McIlroy and Spieth, even though he outplayed them.
“I'm not even close to those guys, Rory and Jordan," said Matsuyama, who finished fifth at last month's Masters.
“I just need to practise more and hopefully soon I'll be able to play at the same level that they do.”
McIlroy, who arrived only on tournament eve after taking a couple of days off following his WGC-Match Play victory on Sunday, continued his good form, his round highlighted by a seven-foot eagle at the par-five 16th.
"It's about staying patient,” McIlroy said of his round.
“There's a lot of pars on my card, but I was able to pick off a couple of birdies and a nice eagle on 16, so pleased with the start.
"I felt conditions got quite tricky out there at the end with the wind. It swirls a lot around here, and made club selection a little bit more difficult."
SPIETH AND WOODS IN DANGER OF MISSING CUT
But Spieth looked a pale shadow of the man who blew away the field at the Masters only four weeks ago, and he admitted he was having trouble trusting his swing after a round that included five bogeys and just two birdies.
Two-time Players champion and 14-time major winner Tiger Woods shot a one-over 73 after finding water on the 18th hole and making a double bogey.
He also double-bogeyed the par-three eighth after a woeful tee shot that found a water hazard way short and right of the green.
(Editing by Andrew Both)
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