Who Can Challenge in the U.S. Open Series?

Given the dominance at the major tournaments by Rafael Nadal, Novak Djokovic, Roger Federer, and Andy Murray, it's a surprise when anyone outside this quartet makes a run at a tournament. So, with that in mind, I aim to look at who could do well during the rest of the U.S. Open Series, which culminates with the U.S. Open itself.

Mardy Fish

The world No. 8 is probably the best of the rest on the hard courts, given his track record and recent form. Last week, he reached the final of the Farmers Classic and despite losing to Ernests Gulbis, he will be looking to have a very promising U.S. Open Series. He's been the runner-up at the Cincinnati Masters twice in his career, including last year. After which, he went onto reach the fourth round of the U.S. Open, though he did reach the quarterfinals in 2008.

Given that he's never beaten Nadal or Djokovic, he probably wouldn't relish playing them, but with one win against Federer, and with questions about Federer's form, he may be more confident going up against the Swiss. Fish also has a winning record against Murray (4-3). In fact, Fish has won the last three of their meetings.

Ernests Gulbis

I'll be honest, I wouldn't have considered putting him on the list had he not won the Farmers Classic. The thing with Gulbis is that he can be brilliant one day and awful the next, but on a good day, he is capable of beating a lot of really good players on tour, as proven last week when he showed some fight having to come from one set down against Fish. Earlier in the tournament, he also ousted former U.S. Open winner Juan Martin del Potro.

Perhaps the appointment of Guillermo Canas may just be the key to unlocking the potential of Gulbis.

Juan Martin del Potro

The former U.S. Open champion is still on the comeback trail, so we may be expecting too much from him, but the top guys won't be looking forward to the thought of playing him. He pushed Djokovic at Roland Garros and Nadal at Wimbledon, demonstrating that he's lost none of the hitting power that he's used in the past to blow opponents off the court.

Losing the quarterfinals to Ernests Gulbis was a shock. Del Potro, however, is strong enough to put the result behind him and move onto the next tournament. In 2009, he surprised us all when he went on an amazing run by winning the Legg Mason, then the Canada Masters, and finally the U.S. Open. Given that and his undoubted talent, he can't be discounted.

Alexandr Dolgopolov

The Ukrainian may prefer clay courts, but his best major performance to date came at the Australian Open earlier this year. En route to the quarterfinals, he beat Jo-Wilfried Tsonga and Robin Soderling in five set encounters. In both matches, he lost the first set. This shows determination to comeback and the stamina to hang in there for such a long time against two of the game's top players.

He's only just coming over to the Unites States after playing a couple of clay court tournaments in Europe, one of which he won. When he does arrive, his fighting style and wide array of impressive shots will most likely light up the crowds. His game is made for a late-night game at Arthur Ashe Stadium.

Grigor Dimitrov

This pick is crazier than Gulbis and I can't even explain why I've picked him, but I just a feeling that Dimitrov will do something.

Obviously some credible challengers, such as Jo-Wilfried Tsonga or Robin Soderling, have been omitted, but I decided to only name five people and wanted a few dark horses to keep an eye on.