Roger Federer enters the 2012 US Open as No. 1 in the world and as the tournament's top seed.

That's the kind of attention that warrants opponents trying to knock him off that pedestal, especially in the final Major of the year.

Over the past couple seasons, Rafael Nadal has been Federer's most formidable foe––facing each other in countless semi-finals and finals all over the world.

Fortunately for the ageing Swiss, Nadal declared himself unable to play the 2012 US Open, citing knee problems, so Roger's chances of winning a record-breaking sixth US Open title may have increased a little bit––maybe. 

Before you all get carried away, Federer will have to worry about a few other challengers that are foaming at the mouth if given their shot to take him down.

Here are the most formidable opponents that Roger will need to pay a little extra attention to in this Major.

Novak Djokovic

Djokovic has to be Federer's most formidable foe being that Novak is the defending champion of the US Open. He's also been No. 1 in the world before, he plays his best tennis arguably on hard-court and in their head-to-head record he's beaten Roger 12 times in their 28 meetings.

Novak has shown a flair for the dramatic and has had a very respectable 2012 season––capturing the Australian Open Championship and racking up a 54-10 win/loss record overall.

Andy Murray

Federer should be very wary of running into a hungry Andy Murray who is still pursuing his first Major Championship at the ripe age of 25-years-old.

Even though Murray and all of Great Britain was defeated by Federer in Wimbledon this year, Murray bounced back weeks later in the London Olympic Games to pick apart Roger to win the Gold Medal for Team GB.

Murray now has the video evidence of how to dethrone Federer. It could be very interesting if the two meet in this tournament. 

Juan Martin Del Potro 

The 6'6" Del Potro has risen back to relevancy here in 2012 and has earned the No. 8 ranking in the 2012 US Open.

Federer may remember the 20-year-old Del Potro from 2009 when the young Argentine took the US Open Championship right from under Fed-Ex's nose.

Del Potro is 48-13 in 2012 and has 2 singles titles to his name.  More importantly perhaps, he appears to have regained the dominance that he displayed three years ago. Delpo could be dangerous, especially on the slick hard courts in Flushing.

These three gentlemen are all playing well heading into the final Major of 2012, they've all had experience beating Federer at one point or another, and all pose a formidable threat to Roger.