Your antagonist is your opponent or enemy. Spassky had never previously lost to his antagonist. your opponent in a competition, battle, quarrel etc protagonist
{i} adversary, opponent; one who opposes; (Pharmacology, Medicine) drug or substance that acts against or annuls the effect of another substance
a drug that neutralizes or counteracts the effects of another drug a muscle that relaxes while another contracts; "when bending the elbow the triceps are the antagonist
A muscle which acts in opposition to another; as a flexor, which bends a part, is the antagonist of an extensor, which extends it