The "incredible mental strength" I'm referring to is after tightening up like crazy in the first TB, he came right out strong in the third set. And then at the end of the fourth, serving for the match, feeling like he was getting jammed up with challenges, Fed yelling something at him, and then losing the set meekly; after all that he came out like gangbusters in the fifth. That's incredible mental strength. Ask Stosur and Amalgro how not winning the match when they were serving for it went.
Part of mental strength, IMO, is when the match doesn't end when you want it to, feel it should, need it to, are praying that it will, and to be able to shake it off and win the next game or the next set. Luckily, because of Murray's fabulous serving he always had a one set cushion.