How vulnerable are the big four?

The big four invincible no more? The lead-up to the U.S. Open revealed a few cracks in their armor, enough to suggest that reaching the final weekend isn't a guarantee. There was Novak Djokovic finally grinding to a halt in the final of the Western and Southern Open in Cincinnati last week. A week earlier, Rafael Nadal and Andy Murray went out in their opening rounds at the Rogers Cup in Montreal. Roger Federer had consistency in his favor but not much else, falling in the quarterfinals of both.

On the other hand, if it's not one of them who walks away with the U.S. Open trophy, then who? Some players can beat anyone on a given day (Gael Monfils, Richard Gasquet, Ernests Gulbis). Others can stay steady and take advantage of gaps or letdowns to go deep into the draw (Mardy Fish, David Ferrer). But not many can do both. Injury means many of the usual outside threats -- Robin Soderling, Juan Martin del Potro, Andy Roddick -- are not heading into New York on a high, while others like Jo-Wilfried Tsonga and Tomas Berdych must maintain their recent form over another two weeks.

So though it's possible for any of the top four to individually be vulnerable in New York, it's hard to see them all falling. Grand Slams, with their extra pressure and best-of-five format, tend to favor the top players, and their recent dominance means the rest of the field has relatively little experience in big finals. Even Murray has found the last hurdle a difficult one to clear, which has led some to question his inclusion in the so-called big four. But when it comes to U.S. Open favorites, he and the other three are leading the pack by a substantial margin.

Here's a look are where each stands heading into the year's final major:

Novak Djokovic

It's the question being asked all season: What is it about Novak Djokovic this year? The Serb has gone from being a consistent threat to becoming virtually unbeatable, and it's hard to tell exactly what's different. Observers and opponents both suggest it's a small but significant improvement in his all-around game, starting with a repaired serve but extending to the greater aggression, purpose and somehow the accuracy with which he is hitting each shot. Confidence and better stamina layered on top of his stable, versatile array of strokes is proving to be a potent formula -- he is, famously, 57-2 in matches this year.

Any speculation that the long break after Wimbledon might interrupt his momentum was promptly squashed as Djokovic began his reign at No. 1 by winning nine matches in a row at the two summer Masters events. But the wins took their toll. His shoulder was already bothering him by the time he lifted the crystal racket trophy in Montreal, and his energy and serving were off all week in Cincinnati before he finally threw in the towel and retired in the final.

It was a classic example of the win-rest tradeoff he has been trying to balance the entire season, and that's why Djokovic's biggest barrier to winning the U.S. Open might be that he's been winning too much. Still, as problems go, is there a better one to have?

With a week of rest, Djokovic says his shoulder should be ready to go in New York -- the only caveat being that he had yet to have full tests done on the injury when he made the statement. Apart from that, it's his to lose. He goes in unchallenged as the best player in the world, and he'll probably have to be a bit tired and error-prone to be vulnerable. Even then, it'll likely take a truly exceptional performance to prevent him from winning his first U.S. Open title.

Rafael Nadal

The defending champion comes into this year's event far away from where he was last year. Then, he was the dominant No. 1 and playing some of the best tennis of his career. This year, he's lost to Djokovic five straight times, including in the Wimbledon final, and then struggled to get going during the summer hard-court events.

Coming in with just a week of practice because of a lingering foot injury, he lost his summer debut in Montreal after serving for the match. Then a tough week in Cincinnati where he badly burned his fingers at a restaurant, leaving him unable to practice his backhand or place his serve properly, and he played 3 hours, 38 minutes in the third round. Exhausted, Nadal lost to Fish in the quarterfinals, leaving him with a grand total of four hard-court matches coming into the U.S. Open -- and a long list of things to work on.

"A little bit of everything. My movements are not being perfect, more mistakes than usual with the forehand. I have to play more inside the court, more aggressive. I have to be ready to play with high intensity and with my best rhythm for four hours. I have to be ready to defend well and I have to be ready to be focused and for sure prepared well mentally to play the important moments of the matches."

Phew. Still, Nadal can comfort himself by remembering that he didn't play his best in the weeks before the U.S. Open last year either, but he suddenly found himself playing aggressively and serving better than ever before. Unlike last year, however, he doesn't have the confidence built up from the early part of the calendar and will be looking to play his way into form during the first couple of rounds. But one thing that won't change is Nadal's effort level, with the Spaniard saying, "I don't know if I will be in perfect conditions in New York, but for sure everything I will try."

Roger Federer

This will be the first Grand Slam of Federer's 30s, and it's not at all clear how it'll go. His game is still there, but not quite as dependable as in the past. He took two quarterfinals losses to big hitters coming in -- one to Tsonga and one to Berdych. Both played well, but Federer looking rather erratic in rallies.

Apart from saying that "physically, I'm feeling perfect," he didn't look or sound particularly happy when assessing his form, either. "It's a quick court," he said after going out in Cincinnati. "So you're not going to be coming out of this tournament feeling like you have the greatest rhythm in the world, especially with the guys I played against. I did play against very strong, tall, aggressive baseline players, you know, I mean, so we were never going to see the longest of rallies.

"But all in all I feel OK. I feel better now than I did in Montreal, where it wasn't a very good match against Tsonga. So at least I have three more matches under my belt."

But, he insisted, "I know what I need to work on."

His comfort on the surface helps, as does the fact that his serve has been in good shape. But whether he can get it all working as well as it was at the French Open remains to be seen, for something close to that will be needed to make a real dent at Flushing Meadows. A little luck from the draw wouldn't hurt.

Andy Murray

Murray has won seven Masters titles and reached three Grand Slam finals -- all on hard courts, no less -- which is why he's consistently included in this group at the top of the game. What he hasn't done is win a Grand Slam, of course. Will this finally be the one? It's hard to count the number of times that's been asked (especially by the British press), and the longer it takes, the harder the task seems to become.

His lack of a truly big weapon is often pointed to as a handicap on such occasions, and for all his other skills, it does mean that it is harder for him to sneak out matches when he's not playing his best. Even when he does play well and get to the later rounds, the challenge can become mental. Murray has struggled to produce good tennis in each of the three Slam finals he's played. But if things come together, he is clearly good enough to win one and is the best player without a major to his name at the moment.

His buildup has been mixed. After losing in the first round at Montreal, Murray managed to get going and win the title in Cincinnati, though he was denied a proper test when Djokovic retired in the final. But he's hopeful it means that, unlike the past few years, his best tennis of the summer is ahead rather than behind.

"In previous years, I've gone in feeling like I was playing really good tennis [and haven't won the U.S. Open]," said Murray in a courtside interview after the final. "I feel like I hit the ball better and better as the week went on. Still a lot of things to work on."

Kamakshi Tandon is a freelance tennis writer for ESPN.com.