Lol they were both scared of each other. If Itachi wasn't afraid of Obito, then he would've killed him along with the rest of the Uchiha considering Obito was the biggest threat back then. :|
Caution and fear are not the same thing. He knew Itachi would give him trouble and be a hinderance, he didn't necessarily think Itachi would get the best of him. (Which he didn't in the end, what does that tell you?)