The hometown star and tournament host shot an 8-under 64 at University Ridge to take a one-stroke lead over 63-year-old Brad Bryant in the PGA Tour Champions event.
But the pair enjoyed a welcome return to form in CT, with Spieth holding a share of the lead with Zach Johnson, while McIlroy was just one shot further back.
Despite missing the cut in last week's US Open at Shinnecock, Spieth remains confident after making the short trip to TPC River Highlands in Cromwell, Connecticut, for today's opening round.
"It's good to see a good score", said Spieth. "I think the PGA Tour, specifically this tournament, does a great job in that". The three that I've won, I came off two seconds going into The Masters. But still a long way to go.
Defending champion Spieth blasted out of a bunker 31 feet for eagle at the par-five sixth, evoking memories of the bunker hole-out the United States star hit past year in a playoff to win the event at TPC River Highlands in Cromwell, Connecticut.
Last week in the U.S. Open, Spieth spot 78-71, 9 over par, and missed the cut at Shinnecock Hills.
"That was a bummer for sure".
"I just need to get my alignment back on the full swing ... but I've got a few weeks off to get back in order".
"I would probably call it third best of the year, behind the first and last round at Augusta". Spieth was delighted to have finally made a good start but is keeping things in perspective.
"Walking up to 18 it was really cool". I hit one in the water a year ago.
"I've just got to be a little more patient and smarter, which is what I've been working on anyway". Last week was, okay, how can I manage myself around here where I have the hard holes you can make pars on and the easy holes I can attack to make birdie.
"That was pretty special", Spieth said. "I'm seeing my lines better and it's a matter of time before it gets to the level I've been at before".
Spieth said the joyful chest-bump celebration with his caddie has gotten more attention than the shot.
Thomas said he, like most others, isn't just using this to tune up their game for the next major. "It drops and you just react". "And also, with all just kind of the subtleties of the slopes and the tree-lined fairways and water areas and trees, it just seems like the wind will swirl a little bit here, and you just have to make sure you know exactly what you're trying to do with the winds".
Costa Rica disappointed after Brazil defeat
Nigeria's pace and solid defending were enough to beat Iceland 2-0, with a double from Ahmed Musa. The thumbs represent the heads of the two eagles, while the fingers look like the feathers.
Xiaomi shares camera samples shot on the Redmi 6 Pro
The other new thing that Xiaomi unveiled today, is the next iteration of its Android tablet line, the Mi Pad 4 . The device will arrive in five colour variants - Rose Gold, Red, Sand Gold, Stone Black and Lake Blue.