The 22-year-old Texan has won two major titles in 2015 and was crowned World No.1 thanks to a second place finish at the PGA Championship earlier this month.
But Spieth will now lose the top spot after failing to make the cut at The Barclays event in New Jersey.
The reigning Masters and U.S. Open champion added a three-over-par 73 Friday to the four-over-par 74 he carded on Thursday to miss the cut by five strokes.
"I'm definitely searching for answers," Spieth said in comments carried by the PGA website when asked to explain his dramatic drop-off in form.
"I don't know exactly what I'm going to do from here as far as how I get prepared for next week (when the tour progresses to the Deutsche Bank Championship at TPC Boston) but I have some time to figure it out."
Spieth's troubles mean Rory McIlroy will return to the summit of the world rankings when they are published next week without lifting a club this weekend.
The Northern Irishman is currently taking time out to recover from ankle surgery after damaging ligaments playing football with friends at the beginning of July.
McIlroy's absence will ensure Spieth can return to World No 1. next weekend should his form improve sufficiently at TPC Boston.
"I've reached that peak already and I know it's going to be close enough to where if I just get the job done next week, I'll be back in that ranking," Spieth continued.
"But again, that ranking, it's great once you reach it but it's not something that I'm going to live or die on each week. It doesn't really make much of a difference.
"If you go on a three- or four-year cycle, Rory is No. 1 in the world. If you go just base off of this year, I am."
As Spieth heads home, Bubba Watson will be looking to extend his lead at the New Jersey course.
The 2012 and 2014 Masters champion leads the field at seven-under-par going into day three.
Open champion Zach Johnson, Henrik Stenson, Jason Dufner and Tony Finau all lie just one shot back on six-under-par.