1. Novak Djokovic- The Serb is the current world No. 1, giving him a big target on his back. Djokovic has a seemingly comfortable lead at over 4500 points, but having that many points means there is plenty to defend. Djokovic is the top-ranked player in the Race rankings, which means he is the favorite to finish the year as the No. 1 player in the world, but two losses in March to players outside the top four has made his lead much less substantial.
Djokovic didn't win any clay court titles last year, but he did reach the finals in three of the four clay events he competed in. He has 2580 points to defend. Madrid will be on red clay again this year, so defending the 180 points there should not be difficult. However, defending 2400 points in Monte Carlo, Rome, and Roland Garros will be extremely difficult since Djokovic could be faced Rafael Nadal in the semifinals this time around.
If Djokovic drop in the rankings to No. 2 in Paris, he will quickly get it back in August. Djokovic only reached the semifinals at Wimbledon in 2012 and will only lose 270 points from the London Olympics.
After that, there is little room to improve for Djokovic, who won titles in Canada, Beijing, Shanghai, and the Barclays ATP World Tour Finals. The only tournaments where he could conceivably improve his ranking are the US Open and Paris.
Djokovic has a ton to defend over the next eight months of his tennis career, but the world No. 1 achieved that title for a reason and is more than capable of defending his points.
2. Roger Federer- The Swiss world No. 2 will have the toughest time of any of the Big Four at trying the overtake Djokovic. However, Federer proved last year that you can never count him out. Federer decided to skip out on Miami, which is perhaps a sign that returning to the top-ranked position in the world isn't that important to him. Also Federer has failed to reach a final so far this year, so if Federer wishes to improve his ranking, he needs to act quickly. Federer's success is evenly spread out over the next five months of the season, so this isn't going to be quick or easy for Roger. Where Federer can do his damage is in the final three months of the season, where he took a back seat to Djokovic on the hard courts.
Federer skipped the event in Canada last year, but even if he reaches the semifinals in Montreal this year, he still will not have made up for the 450 points lost from the London Olympics. So Federer will begin to make his move at the US Open. The key for Federer is to be within range of the world No. 1 when he arrives in New York. I would say that within 3000 would be in range of achieving world No. 1 for Federer. However, that is easier said than done since he is currently trailing Djokovic by over 4500.
Federer must be nearly perfect over the next eight months if he wants to be the world No. 1 at the end of 2013. According to the Race rankings, Federer is the seventh most likely person in the world to be the No. 1 in the offseason, and he will be even lower once the new rankings come out after Miami. Combine that with an expected no-show in Monte Carlo and Federer could be outside the top 10 in Race rankings over a third of the way into the season.
The odds are not in Federer's favor, but he has proven the doubters wrong again and again. Can he do it one more time?
3. Andy Murray- The Scot is Novak Djokovic's biggest immediate threat to the top ranking in the world. At No. 3 in the world Murray is just two wins away in Miami from becoming the No. 2. If that does happen, Murray's chances of surpassing Djokovic increase in two ways. First, he will have 1000 points from Miami, which would be his first ATP Masters Series title since 2011. Second, Murray will be the No. 2 seed at all of the big clay tournaments over the next three months. That means that there is a possibility that Djokovic, Nadal, and Federer will all be on the opposite side of the draw for any of the big upcoming clay tournaments.
If there is a draw where Murray doesn't have to face any of the other Big Four, it would be hard to imagine him not reaching the final. Murray failed to even reach a semifinal at a clay tournament, so gaining ground on Djokovic should not be a problem for the Scot.
Losing in the quarterfinals of Indian Wells was a big missed opportunity for Murray, but he can make up for it with a win in Miami. If Murray wins Miami, he will trail Djokovic by 3620 points. If Djokovic defends all of his points in the clay season, Murray will need to do 3620 points better than he did last year. Murray could possibly add 4780 points to his ranking by the end of Roland Garros, but that is more than extremely unlikely. That means that Murray is going to need help from Djokovic if he wants to become the 26th player to be ranked No. 1.
If Murray has the best clay season of his career and becomes the world No. 1, it will be tough for him to hold onto. Murray will be defending a final in Wimbledon and a US Open titles, while losing 750 points from the London Olympics. Murray can make up for those points at the Masters Series events, where he has plenty of room to improve.
Murray has to play some inspired tennis over the next three months if he wants to achieve a career-high ranking, but even if he fails, he has still one of the top contender for year-end No. 1. He is currently No. 4 in Race rankings, he could pit himself at No. 2, not far behind Djokovic.
4. Rafael Nadal- Who would have seen this coming just a month ago, but Nadal is actually Djokovic's biggest threat to the year-end No. 1 ranking. The Spaniard is ranked No. 2 in the Race rankings despite missing the first major of the year. Nadal missing the Australian Open has actually helped the No. 4 player in the world in terms of ranking. Because he skipped going to Melbourne Nadal decided to compete in three smaller tournaments in South America. At those tournaments, he accumulated 900 points before going to Indian Wells, which was 180 more than he had last year at Indian Wells.
Then at Indian Wells, Nadal reintroduced himself to the tennis world, winning a hard court tournament for the first time since 2010. Skipping Miami, may have put a dent in Nadal's chances at returning to No. 1 in the world, but a good clay court swing will erase that quickly.
Nadal has a ton of points to defend from now until June, but when Wimbledon comes, Rafa will be getting free points at every tournament. For Nadal to become the year-end No. 1, he will likely need to be leading in Race rankings by the time he leaves Paris. After that, if he will have to defend that lead to become the top-ranked player.
In order for Nadal to pass Djokovic, he needs to do as well on clay as he did last year. It won't be easy for the Spaniard, since he won't one of the top two seeds at every clay tournament this year, but there is a reason why Nadal is widely considered the greatest clay-courter of all time. Plus, if Nadal is still trailing Djokovic at the end of 2013, he could be in prime position to surpass Djokovic at the 2014 Australian Open.