Wasn't Kramer career was during and after WWII? That was the time when athletes(especially international sport) are not interested because of the war and post war reconstruction. Even athletes have to join the military to fight for their countries.
Tier1(goat): Federer; Tier 1.5: Laver, Sampras, Nadal, Borg;
Tier 2: Lendl, Connors, Rosewall; Tier 2.5: McEnroe, Agassi