The word ending <sis> is always pronounced sɪs. Here are a few examples:
basis crisis emphasis neurosis thesis
In the plural of words like this the ending is usually pronounced siːz. Thus:
bases crises emphases neuroses theses
Notice that if the spelling <bases> is the plural of basis, it is pronounced ˈbeɪsiːz, but if it is the plural of base, it is pronounced ˈbeɪsɪz or ˈbeɪsəz.