As long as you have the itemId, there is no need to specify the path. For example, this is how to get a specific item within my ODFB site collection:
For SPO site collections it's a bit more complicated, as you need to provide the siteId and listId first, but items within the library can be enumerated by simple incremental numbers. Here's an example:
where I'm getting the 55th item in the Shared Documents library of the default site collection.