)the two boxers battled toe-to-toe until the final round, when the longtime champion of the ring was finally by his young opponent’s stamina.

The answer to this question is "STYMY" which means to hinder or prevent. This happened when the two boxers battled toe-to-toe until the final round comes and when the longtime champion of the ring was finally STYMY by his young and strong opponent's stamina. Until the both of them get weaker and when the final round ends only one win and the other one loses.