Let's face it, Itachi would have beaten Sasuke. If he was not already exhausted, and fought Sasuke as hard as he could... Itachi would have been the victor.
Did you just reach that point in the manga? Itachi could have obviously won that battle but Sasuke had to prove himself, due or die, and Itachi intended for Sasuke to reach him in power and skills, which he did.
Let's face it, Itachi would have beaten Sasuke. If he was not already exhausted, and fought Sasuke as hard as he could... Itachi would have been the victor.