When it comes to the year-end battle for the World No. 1 ranking on the ATP Tour, it looks like a two-player race at this point. Roger Federer and Rafael Nadal, on year-to-date rankings, are No. 2 and No. 1 respectively. Federer has the two Grand Slam titles from the Aussie and Wimbledon to go with titles from Indian Wells and Miami. Meanwhile, Nadal has his French Open title to go with his other clay-court titles. The Spaniard also has a runner-up finish from the Australian Open. When it comes to the year-to-date race, Nadal currently holds a 550-point lead over Federer heading into the North American summer hard-court season.

North America has belonged to Federer

Historically, this point of the season has been more about Federer than Nadal. The Swiss Maestro has won Cincinnati seven times, Canada twice, and the US Open five times. Nadal has won Cincinnati just once, Canada three times, and the US Open twice. After the North American summer, the balance of the season is predominantly about the hard courts as well during the Asian swing. That would seem to suggest a heavy advantage for Federer to finish the year ranked No. 1.

However, the question of who will finish this year top ranked could be about who will be able to play at all as much as who will play well. Both players are injury prone at this stage of their careers.

Last year they both basically shut things down late in the season. Nadal played a little bit post-Wimbledon while Federer didn't play at all. The winner of the year-end race to the World No. 1 ranking might be as much about excellence on the court as it will be about who will be able to appear on the court in the first place for the remainder of the season.

Thiem and Wawrinka have work to do

Importantly, there isn't really a No. 3 player in the picture right now. Dominic Thiem, the 24-year old Austrian, is the third-best player on year-to-date rankings at this point. However, he is a full 3200 points behind Federer. That means that even if Thiem won a Masters Series shield and the US Open, he would still be behind Federer, let alone Nadal.

Furthermore, Thiem winning two tournaments at the 1000 level or better would be out of the blue. The Austrian's best surface is clearly clay, the clay-court season is over, and he didn't even win a Masters Series title or make the French Open final this year. In order to finish number one, Thiem would have to do something on the cement that he couldn't do on clay, the latter being his stronger surface.

Stan Wawrinka is 4th on year-to-date rankings and he's a player that might get himself into the picture to finish the year No. 1. The defending US Open champion brings enough to the hard-court surface that he could beat Nadal and Federer in big matches. Furthermore, the Stanimal has an enduring quality to him that might see him outlast Nadal and/or Federer in terms of participation for the balance of the season.

The problem with Wawrinka is that he is 3395 ranking points behind Federer for 2nd place in year-to-date rankings. Winning the US Open title is the key for Wawrinka just to get back into the debate. He is a player that can't be ignored, but he can't be looked at seriously until mid-September.

Djokovic out of the picture, Murray almost out

With Novak Djokovic shutting things down and with Andy Murray's poor season to date (he's 8th on year-to-date rankings), it appears to be only Federer or Nadal that could finish the year No. 1. Most scenarios would have to suggest Federer as the likely player to finish the season top ranked. If it comes down to hard-court excellence, the Swiss Maestro has out shined Nadal.

If it comes down to injuries and endurance, either player is an equal risk. Nadal might be the one that's had more injuries over the years, but Federer will be 36 years old in a week. He'll have to manage his participation very carefully and that could cost him, keeping in mind that he didn't even compete during the clay-court season.

Attrition and hard-court aptitude combined, the question is if a 31-year old Nadal has more than a 36-year old Federer. As has often been the case with these two players, answering that question is analogous to predicting a coin toss. Tennis fans may have to watch the pre-tournament withdrawal news as much as the scores.