Day's victory at the Players Championship last week was his seventh in his last 17 events, a run which includes his record-breaking major breakthrough at the PGA Championship at Whistling Straits.
Such form has taken Day from 10th in the rankings in June last year to 2.5 points clear of Spieth, with Rory McIlroy a further two points adrift, a marked contrast to the period last season when the top spot changed hands seven times in 12 weeks.
Asked in a press conference ahead of this week's AT&T Byron Nelson if he used Day's recent success as motivation, Spieth said: "Especially playing with him in these kinds of weeks where he's doing what he's doing.
"Going back to Chicago [where Day won the BMW Championship] to Sunday of the PGA to last week, watching the commitment that he had, the focus and then the commitment in each swing that this ball is going where I'm looking and this putt is going in, just stalking it.
"I know that feeling and I'm looking to get that back and definitely there's some motivation there. He's playing his game. He believes his game is better than anybody else's and he's on his game and so it is better than everyone else's.
"What he's doing right now I think I can win the next two events and I'm still not going to surpass him in the world rankings. He's separated himself and that bothers me and it motivates me."
Spieth missed the cut by a shot in the Players Championship in his first event since blowing a five-shot lead with nine holes to play in the Masters.
And the 22-year-old also has a surprisingly poor record on home soil in Texas, failing to record a top-25 finish in the Byron Nelson since a tie for 16th as a 16-year-old amateur in 2010.
"I was on my A-game for all four rounds in 2010 and three out of the four in 2011," the U.S. Open champion added. "I just haven't had my A-game this week since then.
"Do I know why? Not necessarily. One time I think I wasn't driving it well, the next I just didn't make anything. I'm looking for it all to click. I would really like to win this tournament.
"Maybe the first year or two as a professional I put too much pressure on myself to perform but I wouldn't say that was the case last year and it won't be the case this year.
"I feel great about the way I'm hitting the ball right now. I didn't have my putter last week and did some good work with [coach] Cameron [McCormick] the last two days and I seem to have gotten that back. I know it was something simple and it was. So, if we can just continue to do that and roll those putts in, we'll be in business."