EMS Sasuke couldn't do shit against Post-Edo Madara (Not a Juubi Jinchuriki yet)
then
Madara evolves to Juubi Jinchurki
then
8-G Gai almost killed Juubi Jinchuriki Madara..
Yep, pretty sure Itachi has nothing on 8-G Gai.
-Gai knows not to look in any Uchiha's eyes..
-Itachi can be blitzed as shown in his fight against DSM Kabuto
(8-G Gai>>DSM Kabuto in terms of speed)
-Also, he knows who he truly is so Izanami won't work, so....
Maybe, justt maybe the Totsuka Blade will work if Itachi really timed it right. Idk any other way for Itachi to even win.
Gai takes this 10 times out of 10 times.