"I have no idea what the issue was," said Murray.
"He obviously looked like he was in quite a bad way at the beginning of the third set and came back unbelievable at the end of that set.
"Then obviously the way he was hitting the ball in the fourth and moving was impressive."
The Serb claimed he had lost strength in his quads after the gruelling opening to the match.
"I just felt weakness," he said.
"But I couldn't call for a medical (timeout) because there was no reason."
Murray stopped short of declaring that Djokovic had unfairly overplayed any physical distress he was experiencing.
"I would hope that that wouldn't be the case," said Murray, who will rise two spots in the world rankings to fourth next week, while Djokovic further consolidated his hold on top spot.
"But if it was cramp, how he recovered from it, that's a tough thing to recover from and play as well as he did at the end.
"So I'm frustrated at myself for letting that bother me at the beginning of the third set because I was playing well, I had good momentum and then just dropped off for 10 minutes and it got away from me."
The 27-year-old Murray was determined this would be the last time he became distracted by his opponent in such a way.
"In all matches you concentrate on your own end of the court," he said.
"That's just a basic thing to do.
"Sometimes when you play, like here and the US Open, because they have replays after every single point, you're often waiting before you serve whilst they're showing the replay and it's very difficult to not be aware of what's happening down the other end.
"But, yeah, I play enough matches to be able to handle that situation better.
"That's what I'm saying."
- AAP
