American young gun Spieth surged five strokes clear with a six-under-par 66 in the second round, his tournament record low of 14-under 130 leaving Rose and Casey seven shots off the pace in a tie for third.
"Hard not to be aware of what a special couple of rounds he's put in," said 2013 U.S. Open champion Rose, who recovered from three bogeys in his first four holes to shoot a two-under 70.
"The golf course really isn't playing that easy. I know we've had it a little bit soft, but there was some bounce in it today and there's enough breeze where it's tricky.
"And we've played every inch of this golf course today. The tees were way, way back. That's some special golf he's put in."
Rose, who has recorded two top-10s in nine previous appearances at the Masters, was not yet prepared to anoint Spieth as the 2015 champion.
"There's never a big enough lead on this golf course," said the 34-year-old Englishman. "You get to Amen Corner (holes 11, 12 and 13) on Sunday, and there's a lot that can happen.
"Obviously he's in complete control of his game. But for the chasing pack, there's always a score on this golf course, especially on Sunday.
"There's always that 65 out there with an opportunity to make up some ground. It's a major championship. It's never easy playing with the lead, either."
Casey, like Rose, has twice posted top-10s at Augusta National and he was "ecstatic" after shooting a flawless four-under-par 68 in the second round.
"I've played wonderful golf," said the 37-year-old. "I struck it better today than yesterday. I'm ecstatic I'm playing so well, but what Jordan has done out there is brilliant."
Heading into the weekend seven strokes off the lead gave Casey a sense of relaxation.
"You have to aim at flags and take it on and push a little bit more," he said.
"Jordan is sticking a target out there for everyone to chase, and I'm feeling no pressure because I'm halfway there."
Europe has failed to produce a Masters winner since Jose Maria Olazabal of Spain in 1999.
(Editing by Greg Stutchbury)
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