After an astonishing run of form in 2014, in which he won his third and fourth majors at The Open and PGA Championship as well as victory at the BMW PGA Championship and WGC-Bridgestone Invitational, McIlroy stalled the following year, which included a spell on the sidelines with a foot injury that ruled him out of his Open defence.
Spieth, whose second-place finish at the PGA Championship in August saw him replace McIlroy as world No.1, is likely to be partnered with the Northern Irishman and Rickie Fowler at this week's Abu Dhabi Championship -- and he expects the anticipated rivalry to really take hold in 2016.
"In a season that he considered lost, he still came back and ended up winning the Race to Dubai and that proves what a player he is," Speith said.
"I'm sure there are very few players who have been working harder than he is to make this season his best season yet -- and that is scary. Hopefully I can prevent that to an extent.
As Spieth looks to build on his breakthrough season, which saw him slip into the Masters' green jacket in April before lifting the U.S. Open trophy two months later, he admits he would "like to contend in a couple of majors on Sunday and to win at least one of them."
He added: "I'd also like to represent the United States in not only the Olympics but also the Ryder Cup. It would be great to get a couple of wins, not only for myself but for my country."