Johnson then won the first sudden-death hole and speaking afterwards said he is delighted to be back at the top of his game.
"I feel like the game is finally back in form like it was before the Masters. Getting a win here gives me a lot of confidence going into next week and the rest of the play-offs".
Speaking afterwards Open Champion Spieth said: "I didn't lose this tournament".
If Johnson unnerves competitors with his driver, Spieth unsettles them with his putter.
"That's what I feel like is happening now".
The 10-time PGA TOUR victor and former THE PLAYERS champion was left to lament a few stretches of bad holes at THE NORTHERN TRUST, particularly a front-nine 40 on Friday. Kuchar ran off four straight birdies and kept right on rolling, ending his round of 64 with eight birdies on the last 10 holes. "I had a clumsy start and didn't really hit it close enough and wasn't quite firing in the first seven holes".
"Finally, this is the first week I felt like my game was back to where it was; where I feel like I've got complete control over it", said Johnson.
His lead would have been a stroke greater but for Johnson's birdie on the 18th, where he stuck his second shot four feet from the pin to cap a 67 for 201. A missed short putt at the ninth equated to a bogey which, coupled with back-to-back birds from Johnson, erased Spieth's seemingly insurmountable advantage.
When asked after the round of what proved to be the defining shot from the tee Johnson was his typical nonchalant self.
As seen at Hartford and Southport earlier this year (to say nothing of a certain 2016 major), these turbulent final rounds are becoming Spieth's calling card. A risky, but successful opening drive over the lake on the hole set up Johnson for a smooth wedge drive and an effortless putt to take the win in Old Westbury. "I'm like, 'Does that really still have a chance?' And it came around and lipped in, and my initial thought was 'I just did that exact same thing to him the hole before'".
In a career known for driving prowess and field-lapping abilities, it might be the defining putt of his life so far, the most important one he has made when he has really had to make one.
"When someone is playing as well as Julian was, you know that you are going to have make birdies and it really focuses you on what you have to do".
Jon Rahm and Jhonnattan Vegas tied for third at minus-9. "I'm much more mentally sharp right now than I have been all year, which is great, even though I am making silly errors".