From the top of my mind, Murray has played not-so-few matches over several months, so even a man of his exceptional fitness ought to be a bit extra tired by now. A 3:20 match before the prospect of playing tough matches Friday through Sunday doesn't help then.
As for Murray being on the ropes, I think that's very much an accurate statement. Stan can very well beat anyone if he's on, and he was today. So it's bring it or (probably) go home for Murray.
And not only is Murray on the ropes. Nishikori is too. And Stan.
We're basically getting a brutal fight for survival among three hungry and cornered carnivores