Australian Jason Day (72), who also missed the cut last week in the latest misfire of what has so far been an underwhelming season, has plenty of work ahead if he plans to play the weekend after mixing a double-bogey with three bogeys and three birdies.
Defending champion Russell Knox finished one-under 69.
Spieth has already switched putters twice this season but his current flatstick was working just fine on Thursday, the world number six snagging five birdies in his first eight holes en route to a seven-under-par 63 at TPC River Highlands.
He closed his round in style with birdies on three of his final six holes, including at the par-four 18th where his wedge from 136 yards stopped within 5 feet of the hole, to take a slender lead over Johnson Wagner and Brett Stegmaier.
"We played the practice round and the pro-am and really thought this was a good track for us. Sometimes you just fall into that," said Spieth.
Spieth's putting prowess spurred much of the success early in his career but the 23-year-old has gone cold on the greens this year, most recently last week at the U.S. Open where he finished in a share of 35th place.
But the Texan, seeking his second PGA Tour win of the season, looked more than comfortable on his first trek around the TPC River Highlands layout where he had it all clicking and equalled his low round of the year.
Northern Irishman McIlroy started on the back nine where he collected two of his four birdies. He snared a 15-footer at the par-four 17th but followed up with a bogey, his only miscue on the greens all day, when he missed a three-footer for par.
Among those level with McIlroy are Patrick Reed, Swede David Lingmerth and Jim Furyk, whose final-round 58 last year at the Travelers is the lowest round on the PGA Tour.
(Reporting by Frank Pingue in Toronto; Editing by Peter Rutherford)
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