The world No.3 proved he is at the peak of fitness with his straight sets defeat yesterday of Joao Sousa in a Melbourne heatwave.
Murray destroyed the challenge of the Portuguese player yesterday in temperatures of 40.4 Celsius.
And the Scot admitted the top players have had to beef up their six packs with intense gymwork to remain the sport's alpha males.
"Rafa Nadal was the first one to have that physique that looked like a true world-class athlete across any sport," said Murray.
"Tennis players are always in good shape, but I think he looked like he could do any sport. The game has changed for sure physically, much more demanding and I have adapted my preparation and training, reducing the number of tournaments I play.
"The top players rely much more on good, upper body strength. That's vital nowadays and can give you that all-important edge."
Murray is now fully focused on tomorrow's third-round clash with Ricardas Berankis, of Lithuania.
A junior world No.1 in 2007, Berankis is set to break into the top 100 after this tournament. He and Murray know each other well, having practised together on many occasions.
Murray is happy to see him doing well, but said it was no surprise young players were taking longer to make it these days.
"The average age of the top 100 is 28," Murray said. "I would guess that is a couple of years older than five years ago.
"The game is changing, it takes guys longer to physically and mentally cope. It will take guys longer to break through unless you get someone who is incredibly gifted and talented."
Meanwhile, Nadal will return from injury at the VTR Open in Chile, starting on February 4.