Shisui was ordered to keep an eye on Itachi so Itachi was even a little weary of Shisui before his eye was snatched.
Itachi would be able to defend against it (KA) because Itachi wasn't suicidal.. until he wanted to be killed by Sasuke.
However Itachi admired Shisui so he would have been affected by KA pretty severely. & prob do almost whatever Shisui told him to do, if it made sense.
Itachi was weary of the Uchiha's history so he would know anything about Uchiha trying to entice him into anything *wrong* that he wouldn't agree with normally.
but Shisui could have beaten Itachi back then .. because he had MS/flicker but Itachi was a genius as well.
Itachi didn't have MS so he would have lost... o__o