McIlroy placed one hand on the claret jug on Saturday after carding a 68 this afternoon to move six clear of Rickie Fowler and seven strokes ahead of Garcia.
"If Rory plays the way he has been playing it is difficult to see any way to catch him," he told reporters.
"If he shoots three under I have to shoot 10 [under]. It is an Open Championship and no-one's done it before so it's not that easy.
"If he plays the par fives well like he did today it is going to be tough as even though he started slowly he really got going on the back nine."
Victory for McIlroy would hand the 25-year-old his third major following a US Open triumph in 2011 and a PGA Championship win a year later.