As Sasuke came after him, Itachi continued to retreat. The hand signs his brother formed were the most familiar of any he knew, and he was quick to mimic them. They weren't close enough to water yet for him to fight off the flames that way, so canceling them out with (hopefully) equal force would have to do.

He took a deep breath, the heat flowing up from his lungs and leaving his mouth in a burst of flames. Despite the fact that his brother didn't believe he was real, and despite that the other was coming at him with killing intent (something he expected to face one day anyway) he was filled with a great deal of pride at how far Sasuke had progressed skill-wise. These were high end Uchiha techniques and he had made them his own. That was just how it should be.