Shisui was 18-24 when he died while Itachi was 13. Then Shisui was shown to be the type of ninja that completely lets down his guard when he thinks he's one. He also has one OP genjutsu that he can only use once a decade but sets his MS on cooldown for ten years. He's not anywhere near Itachi's level.
No, he wasn't. He just had on OP genjutsu that would set his MS on cooldown for ten years right after using it once. He had the most powerful genjutsu, but he wasn't the best user. By feats and what we've seen so far, that would have to be Itachi considering that he can do what Shisui can do, but better minus having the KA oneshot.
Shisui awakened one OP technique, which was the same case with Obito. Not all the Uchiha are the same and it's been stated that awakening susanno is rare, even for MS users, which is why Itachi was surprised when Sasuke awakened it. Also, you have to awaken the other two powers in order to awaken the third, susanno. However, Shisui only had KA, meaning he did not attain susanno.
ot: Shisui just blitzes taka Sasuke and wins. Sasuke was fooled by one of Itachi's genjutsu crows for a good minute during their fight and Shisui was a pretty adept genutsu user. All he'd have to do is distract Sasuke for a few seconds and then blitz him.