Competitor, n.
Etymology: [L.: cf. F. compétiteur.]
1. One who seeks what another seeks, or claims what another claims; one who competes; a rival. And can not brook competitors in love. Shak.
2. An associate; a confederate. [Obs.] Every hour more competitors Flock to their aid, and still their power increaseth. Shak.
Shades of meaning
Fernald has no entry of its own for competitor; he lists it under enemy.
Where he writes about it:
An antagonist is one who opposes and is opposed actively and with intensity of effort; an opponent, one in whom the attitude of resistance is the more prominent; a competitor, one who seeks the same object for which another is striving; antagonists in wrestling, competitors in business, opponents in debate may contend with no personal ill will; rivals in love, ambition, etc., rarely avoid inimical feeling. — from enemy