Santiago is old, but he is still in good physical shape for a man his age. He has spent his whole life on the water, fighting the winds, the tides, and various fish. Because of his pride, he believes that he is able to overcome any obstacle with his will alone; he only starts to doubt his abilities as his body threatens to betray him. The marlin, which is enormous and powerful, is the ultimate test of his skill as a fisherman; Santiago knows that he doesn't need to be as strong as the marlin as long as he is smarter than the marlin. This is foreshadowed early, when he speaks with Manolin, the boy:
"There are many good fishermen and some great ones. But there is only you."
"Thank you. You make me happy. I hope no fish will come along so great that he will prove us wrong."
"There is no such fish if you are still strong as you say."
"I may not be as strong as I think," the old man said. "But I know many tricks and I have resolution."
(Hemingway, The Old Man and the Sea, Google Books)
As it turns out, Santiago is not as strong as he had hoped. His body slowly weakens, and he can't bounce back as easily as he had in his youth. The strain of fighting the fish starts to break his body and then his resolve. In the end, the only thing keeping Santiago going is his willpower, which by itself couldn't be enough to land the fish, and proves insufficient to fight off the sharks. Santiago fought the marlin until it weakened and allowed him to kill it; the sea then fought Santiago until he weakened and allowed it to "destroy him, but not to defeat him."