ANDY MURRAY needs to "step up" at the US Open if he wants to end his dismal run of form, says John McEnroe.

Heading into the last grand slam of the year on Monday, Murray is looking for his first final since his Wimbledon triumph last year.

The Scot has struggled to find form since back surgery last September.

McEnroe, who will return to London for the Statoil Masters at the Royal Albert Hall in December, said: "It's difficult to say what's happening with Andy.

"It seems the physical issues are behind him.It's a process. Coming back from injury takes its toll on the confidence.

"You feel the US Open would be a time when Andy needs to step up because it's been a struggle compared to the previous two years."

Murray ended his successful partnership with coach Ivan Lendl in March. The US Open will be his second grand slam with Amelie Mauresmo.

McEnroe is reserving judgment on whether their partnership can be a success. The American, who was briefly linked with the role, said: "Andy made a big move. I had thought he and Ivan worked well together.

"Amelie was an unusual choice, but she's had some coaching experience and has been out there, so she can bring things to the table.

"You always need to give these things some time to see how they relate to each other and whether she can make that little difference.

"Can she help him find that extra two to five per cent? That's the key. It remains to be seen."

"To change course when he did is tricky. It remains to be seen what happens there. You need to give it more time."

Murray's chances have been helped by the withdrawal through injury of defending champion Rafael Nadal.

The 2012 winner will be eighth seed, which means in the last eight he could play world No.1 Novak Djokovic or Roger Federer, who beat him in Cincinnati last week.

Djokovic has struggled on the US hard courts this year but McEnroe tips him to take the title.

"He had an incredible run at Wimbledon and it seems to me Novak has been waiting for the US Open," he said. "Recent results don't matter a lot, he's still favourite.

"There are others who will want to break through to put their names in the record books. The most obvious ones are Grigor Dimitrov and Milos Raonic."

Statoil Masters Tennis is at Royal Albert Hall, London, December 3-7. Tickets: www.Statoilmasterstennis.com.