The Collaborative International Dictionary of English v.0.48:

Competitor \Com*pet"i*tor\, n. [L.: cf. F. comp['e]titeur.]
   1. One who seeks what another seeks, or claims what another
      claims; one who competes; a rival.
      [1913 Webster]

            And can not brook competitors in love. --Shak.
      [1913 Webster]

   2. An associate; a confederate. [Obs.]
      [1913 Webster]

            Every hour more competitors
            Flock to their aid, and still their power
            increaseth.                           --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

competitor
    n 1: the contestant you hope to defeat; "he had respect for his
         rivals"; "he wanted to know what the competition was doing"
         [syn: rival, challenger, competitor, competition,
         contender]

Moby Thesaurus II by Grady Ward, 1.0:

106 Moby Thesaurus words for "competitor":
   adversary, amateur athlete, antagonist, archer, athlete,
   ballplayer, baseballer, baseman, batter, battery, battler,
   belligerent, belted knight, bickerer, blade, blocking back, bowman,
   bravo, brawler, bully, bullyboy, catcher, center, coach, combatant,
   competition, contender, contestant, corrival, cricketer,
   defensive lineman, disputant, duelist, emulator, end, enforcer,
   entrant, fencer, feuder, fighter, fighting cock, foilsman,
   footballer, gamecock, games-player, gamester, gladiator, goon,
   gorilla, guard, hatchet man, hood, hoodlum, hooligan, infielder,
   jock, jouster, jumper, knight, lineman, militant,
   offensive lineman, opponent, opposition, outfield, outfielder,
   player, plug-ugly, poloist, professional athlete, pugilist,
   quarreler, quarterback, racer, rioter, rival, rough, rowdy,
   ruffian, sabreur, scrapper, scuffler, skater, sport, sportsman,
   squabbler, strong arm, strong-arm man, strong-armer, struggler,
   swashbuckler, sword, swordplayer, swordsman, tackle, tailback,
   the field, thug, tilter, tough, toxophilite, tussler, vier,
   wingback, wrangler, wrestler