The Scot claimed his first Grand Slam title at Flushing Meadows with a five-set victory over Novak Djokovic in the final in September, in what was his fifth major final.
Croft believes that Murray is as hungry as ever following his triumph in New York.
"He's so at ease," she told thetennisspace.com. "I heard that he feels he just doesn't have anything to prove anymore.
"No-one's going to keep asking him about when he's going to win a Grand Slam. He's at ease with himself. And from what I hear, that training block in Miami, he worked even harder, because he knows it paid off.
"It's funny even reading the local papers here, there's a lot of them talking about him. He focuses forwards rather than blaming backwards. And that's gained him so much, because you don't feel you can get under his skin so easily. It's really interesting how he's developed."
Murray faces Jeremy Chardy in the quarter-final of the Australian Open.