As far as we know Tobi isn't much of a genjutsu user, in fact I can't think of a time he has ever used genjutsu. This leads me to believe he's not very skilled with it. If itachi actually managed to trap Tobi in a genjutsu then it might give him the time he needs to finish him off.
However I think itachi would need a way to combat Tobis powerful space/time ninjutsu, susanoo' might work, unless Tobi warped inside the surrounding chakra and absorbed itachi into his eye.
Though some have said that Tobis eye warping technique puts the user in a genjutsu. Though this is just speculation, I think itachi might be the one person who could find a way out if that is the case seeing as he is a genjutsu master.
Honestly it's hard to tell who takes this fight, izanami is a huge risk, and I don't think should be considered as an option in this fight. Itachi does not know enough about Tobi to risk using it, and I don't think he would.
IMO this fight has a lot of different variables, too many to declare a real winner.