The athlete’s lips curled disdainfully, and without honouring his adversary with a formal denial, he exhibited, as if by accident, that peculiarly Russian object—an enormous fist, clenched, muscular, and covered with red hairs!
There are no more uses of "adversary" in the book.
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Then, again, our military training is in many respects superior to that of our adversaries.
Thucydides -- Pericles’s Funeral Oration
Finally, it was rumored, there would be complete information-sharing among all of these previously disconnected and even adversarial entities, and when they were coordinated, and once all the health data they’d collected was shared, most of this made possible through the Circle and more important, TruYou, viruses could be stopped at their sources, diseases would be tracked to their roots.