Criminal Sentence 214: "Nemesis": Singular or Plural?

Said by a baseball announcer:

"Their nemesis have been the opposing pitchers."

If the words were "nemesi" (singular) and "nemesis" (plural), this would have some chance of being correct. However, the singular is "nemesis" and the plural is "nemeses." The subject ("nemesis") is singular, so the sentence should be as follows:

"Their nemesis has been the opposing pitchers."

The sentence would sound even better if we made the object ("pitchers") singular:

"Their nemesis has been the pitching of the opposition." (or "the opposition's pitching")