He may have really powerful chakra, but there is nothing stating that quantity wise, he has a lot of it.
In a fairly detailed explanation, Ebisu stated that chakra is converted from stamina, so it goes hand in hand.
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Meaning that if Itachi in Part I had a 2.5 in stamina, then he's rather limited in the chakra department.
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Here, after Itachi used Tsukuyomi on Sasuke and Amaterasu to escape Jiraiya's fire breathing stomach toad jutsu, we can see he's visibly worn out.
You could say he was faking it in order to not have to fight Jiraiya, who was a Leaf Shinobi (and thus, a member of a village that Itachi was trying to protect).
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Here, after previously having had used Tsukuyomi on Sasuke, and now using Amaterasu, you can say he's visibly tired. (We can't count this TOO much either, because at this point he was known to be sick).
However, just because a jutsu is devastating doesn't mean it has to be the most chakra taxing technique ever.
Kakashi, who has a "barely average" level of chakra was able to use Chidori in Part I four times.
But is there anything stating that Chidori takes less chakra than Amaterasu? It could very well be that Amaterasu is more destructive because of a more advanced shape manipulation.
I'd say that Itachi pre-illness had a weakness of chakra.
But in the end, he had more than enough control to exercise it efficiently and also the intelligence to know WHEN to use the technique. In Part I, until we see something that tells us otherwise, we can only say that he did not have the illness.
EDIT: The guy above me more-or-less beat me to it.
But at the very least, two members agree to this.