I think this is a great topic.

One that comes to mind is Ali - Foreman

At this point Ali had lossed twice and was the underdog probably for the first time in his life since Liston. Foreman was an unstopable force who had tottally destroyed and demolished both men who beat Ali. After the fight, Ali's career got a HUGE seccond wind. Had Ali Lossed that fight I don't think he would be what he is today. It is quite possible that he would not have finished the triology with Frazier or gotten the re-match with Norton. Foreman in the other hand never recovered and had to wait an incredible 20 years before winning a title.