Del Potro had an 18-6 losing record to Roger Federer before Sunday's showpiece but he got out the blocks fast.
The Argentine took the first set in just 34 minutes despite Shiras claiming Del Potro looked 'half dead'.
While speaking on Sky Sports commentary, Shiras said: "He has such a characteristic walking style. He looks like he's half dead from the start of the match.
"I think he's in the right frame of mind. He has that languid feel about him and you can't tell if he's energised or not.
"But he operates at his own tempo and once he starts moving he's a really brilliant mover.
"You wouldn't know it watching him walk in at 10am in the morning.
"He's always a little bit slow going but he's a great defender and one of the best attackers in the game so Roger has to deal with that."