Why didn't Itachi fight Madara

[Nick01]

Would've been a good fight, though Madara would've still won.

 

[Obito the supreme jin]

Itachi was still under the assumption that Tobi was Madara so he left Naruto that task and went to undo the Edo Tensei which was the Akatsuki's major strength. Itachi put a huge dent into their plans because of this.