Este é provavelmente o conto ' Arena ' por Brown Frederico .
Como mencionado na resposta de @ beichst, um episódio de Star Trek TOS também chamou Arena foi escrito com notáveis semelhanças com este conto, embora não se pretenda que este conto seja a base. Brown recebeu mais tarde o crédito pelo teleplay.
Ele apresenta uma batalha galáctica entre a humanidade e uma misteriosa raça chamada 'The Outsiders' quando outra raça de maiores poderes intervém!
Aqui faz parte do resumo da trama da Wikipedia, eu destaquei os bits importantes referentes à pergunta:
The mysterious Outsiders have skirmished with Earth's space colonies and starships. Their vessels are found to be faster and more maneuverable, but less well armed. There have been no survivors of the small raids on Earth forces so Earth has no information about the Outsiders. Fearing the worst, Earth builds a war fleet. Scouts report a large armada approaching the solar system. Earth's defenders go to meet them. All indications are that the two fleets are evenly matched.
Bob Carson is the pilot of a small one-man scout ship on the outskirts of the fleet. While engaging his Outsider counterpart in battle, he blacks out. When he awakens, he finds himself naked in a small enclosed, circular area about 250 yards (230 m) across. ..
Carson hears a voice in his mind that identifies itself as the end product of the evolution of an entire race. While traveling through various spaces and dimensions, it had come upon the impending battle. The evolved intelligence decided to intervene because both humans and Rollers have the potential to one day evolve into a being like itself, but the upcoming war would utterly destroy one side and hurt the other so badly that it would not be able to fulfill its destiny. It therefore chose one individual from each species to fight in this small arena. The loser will doom its kind to instant extinction.
Carson and his opponent discover, through trial and error, that there is an invisible barrier between them, and that living things cannot cross it, though inanimate objects can. Carson tries to communicate with the Roller, to see if a compromise is possible, but receives a mental message of unremitting hatred.