Oddsmaker Bovada is giving McIlroy (No. 1 in the world) 7-1 odds to win the U.S. Open, which begins Thursday, with Spieth (No. 2) right behind him at 8-1.
"There's going to be someone lifting the trophy at the end of the week," McIlroy told the Associated Press. "It's a bit of an unknown to most people, so you have to prepare. But you can fall into the trap of trying to over-prepare."
Spieth is one of the few competitors with experience at the University Place, Wash., course, which has been hyped as one of the most difficult ever to host the U.S. Open, though not all of it good. In 2010, he withdrew from the U.S. Amateur Championship after shooting an 11-over-par 83 in the second round.
However, Spieth forged a happier memory there in 2013 at his caddie Michael Greller's wedding on the course.
"Players that have played there, especially in the 2010 U.S. Amateur, are going to be at a monumental advantage, about how to position certain shots, how to play certain holes,"
Said Spieth: "I didn't really see much of it. Actually, I saw a lot of it — I didn't see much of the places I want to see."
Other favorites to win include
Reigning U.S. Open champion