here is my interpretation of the Itachi vs Kakashi fight

First you need to realize that Itachi does not keep his arms in his sleeves. This is shown when Itachi reveals himself using his hand from the inside of the coat to open the front. He keeps his hands inside. The reason we did not see him performing hand seals when he attacked Kakashi is because he was doing it inside his coat. He whipped out the shurikens in his hand after performing the seals to fake Kakashi out. Kakashi would have been expecting a shuriken attack when actually the water jutsu was already performed.

There is only one person that has existed in the Naruto Universe they could perform Jutsus with one hand, that person being Haku. Even Kakashi said it was unheard of.

-take