Overview:June 28, Pacquiao will try to add his fourth world title to a resume that has left him universally rated the best pound-for-pound fighter in the world, now that Floyd Mayweather Jr. has -- at least for the moment -- left the stage. Yet as accomplished as Pacquiao has become, all men have their limits. The question is: Has Pacquiao finally reached his? When Pacquiao slips into the ring to challenge WBC lightweight champion David Diaz, he will be facing a 135-pound opponent who began his career weighing 142 and has worked down from there. In other words, when these two first became professionals there were 36 pounds between them.