Wimbledon champion Andy Murray must get used to being "the hunted" as he prepares to defend his US Open title, says John McEnroe.
The American, a four-time winner at Flushing Meadows, believes that as the reigning champion Murray, 26, faces a different kind of pressure when the tournament begins on Monday.
"On the one hand he's done it, so that's amazing," said McEnroe.
"On the other hand, you're the hunted more than you were in the past."
Murray, 26, has reached the final of the last four major tournaments in which he has competed, but has not made it past the quarter-finals in his two events since his victory at Wimbledon.
He remains in with a chance of ending the year as the world number one but slipped behind the in-form Rafael Nadal to third in the rankings on Monday.
McEnroe, 54, managed to follow Wimbledon victories with success at the US Open twice during his career, in 1981 and 1984, and both times ended the year as the world number one.
"This is the first shot where Murray's got a legitimate chance to finish number one and be number one for the year if he can get two majors," added McEnroe, now a regular on the ATP Champions Tour that culminates at the Royal Albert Hall in December.
"So that's a different type of approach and people are going to be gunning for him even more. I think he's found that a little bit since playing Wimbledon."
McEnroe also believes the fact that Novak Djokovic won the Australian Open, Nadal won the French Open and Murray won Wimbledon, means the US Open could effectively decide who has been the best player of 2013.
"There's three different guys, obviously Rafa, Novak and Andy, that have won the three majors," said McEnroe.
"All three of them feel that if they win the US Open - I think - that they're going to deserve to be called number one for the year."
The US Open has been disrupted by bad weather for the past five years, with the men's final being postponed to a third Monday on each occasion, and McEnroe has welcomed the news of plans for a roof over Arthur Ashe Stadium.
"I don't want to say too little too late because I don't think it ever is," said McEnroe.
"It would fall under the category of better late than never. It's been five years in a row, which has hurt the ratings and hurt, I think, our sport a little bit, that our final was not played on the Sunday.
"Clearly that's bad luck, but at the same time the USTA [United States Tennis Association] makes a lot of money at this event, it's a huge event, and they have the resources, so I'm glad they seem to have finally figured out a way to put a roof over a huge stadium."