Working the body and using superior hand speed to land big combinations, Ruiz finished his handiwork with a TKO four rounds later, shocking the reigning champ to become the first fighter of Mexican descent to capture the WBO, IBF, WBA and IBO world heavyweight titles.
Ruiz has a stated goal weight of 255 pounds, or 13 pounds less than his fighting weight against Joshua.
"He fights like a smaller man because of his hand speed, his ability to throw combinations," said Joe Goossen, a longtime trainer and commentator for Premier Boxing Champions on Fox.
Even small changes in body composition can markedly affect the body's energy needs.
That left some experts wondering whether even 10 or 15 pounds was too much weight to attempt to lose before the December 7 rematch.