Seriously, how did Rocky himself not die like Apollo from fighting Drago?

Ivan Drago was not only younger than Rocky (as well as Apollo Creed), but he was taller, stronger, and faster. Plus, he was juiced to the gills. I know that they tried to work around that by going with the brain damage angle in Rocky V. But still, how was Rocky Balboa able to last 15 rounds with a guy who literally killed his previous opponent in the ring in short order?

And Dolph Lundgren legitimately hurt Sylvester Stallone when they were shooting the final fight scene. If I have my facts straight, Stallone told Lundgren to make it look as real as possible and not something obviously choreographed or pantomimed.