At the beginning of the initial fight when your opponent is approaching you, use your special power punch. This will stagger him. When he approaches you again, hit him with another special power punch, then attack him with an array of body punches. His strength will drop. Keep attacking him to knock him down and thus win the fight.
Max out your speed first, then your strength. This helps in the matches because all you have to do is keep throwing hooks, then when your opponent is dazed, use your Super Punch to knock him down. After you have maxed out speed and strength, you should be able to defeat all of your opponents with ease using hooks.