Do you not get the fact everything takes time, nothing is instant, it can be fast but it still takes TIME. Minato is known to move a SHIT load faster than Itachi can raise a finger + mold chakra at the same time, or did you forget the basics of Naruto? Every jutsu requires chakra molding. SOOO lets say Itachi takes 2 seconds to raise a hand and mold chakra to cast genjutsu, in this time Minato has seen Itachi moving and reacted in the milliseconds that reaction occurs and he then "flashes" (for lack of a better word) to a seal, in this time moved out of Itachi's vision = no genjutsu/amaterasu, there was a reason his speed was regarded as his greatest asset.