Target: Novak

Djokovic has been No. 1 since July 4, when he switched spots with Nadal. Glyn Kirk/AFP/Getty Images

FROM HIS GLUTEN-FREE DIET to the CVAC pod he reportedly used to jack up his red blood cells, Novak Djokovic spent the past two years re-engineering himself. The result? A 70-6 record in 2011 that made everyone else look slow. While Djokovic credits confidence, not training, for his wins, the real question is whether his rebuilt game is built to last. The Serb staggered to the end of his historic year, pulling out of the Paris Masters in the quarterfinals with a bad shoulder in November and then losing in the ATP World Tour Finals to David Ferrer two weeks later. "My body says it's overloaded," he conceded. Less sliding and taking returns a little later can save Djokovic some wear and tear, but he will have to fight to keep his top spot, especially if his rivals can make a few key adjustments.

Roger Federer (No. 3 in the world) has the best record against Djokovic (14-10) and the least respect for his style of play. After Djokovic returned a match point against in the U.S. Open with a cross-court crusher that led him to victory, Federer was almost haughty in defeat. "Are you kidding me?" he balked. Djokovic is a master of redirecting apparent winners, so Federer will need to match him surprise for surprise, wrong-footing Djokovic wherever he can. He also has to find the serve that made him No. 1 for 237 straight weeks, not the one that trailed from weary legs in Flushing. A word of warning to Novak: Federer took six weeks off after the Davis Cup in September and is taking a 17-match winning streak into 2012.

Rafael Nadal (No. 2) has 10 Grand Slam titles courtesy of his topspin bombs that force opponents back and make them hit short returns. In response, Djokovic spent 2011 stepping closer to the baseline and putting Nadal on the defensive. It worked. Djokovic spent last year sending balls where Nadal has never seen them go, defeating the Spaniard six times. Nadal spent November talking about getting his motivation back, but he'll need to shorten points against Djokovic. In that high-torque matchup, the best shoulders and knees will win.

Andy Murray (No. 4) appeared to be on a roll with one Slam final (Australian) and three semis (French, Wimbledon and U.S.). Emulating Djokovic, he went gluten-free in August. He then won 13 matches in 18 days on the Asian leg of the ATP Tour, only to pull out of the World Finals with a groin injury. The buzz now is who will be his new coach. Ivan Lendl and Jimmy Connors were names being floated. Both can teach Murray what he really needs to copy from Djokovic -- confidence.

Yes, Ferrer (No. 5) beat Djokovic at the World Finals, but credit Djokovic's 33 unforced errors as much as Ferrer's speed and consistency. Against a fresher Djokovic, Ferrer will have to make his serve a weapon instead of the start of a long point.

Jo-Wilfried Tsonga (No. 6) is a crowd favorite for his high-risk, high-reward attack. He fell to Djokovic at Wimbledon, but adding just 10 percent more consistency to his return game will make him trouble for the Serb -- assuming that Djokovic hasn't already rebuilt himself again.

Shaun Assael is s senior writer for ESPN The Magazine. Follow ESPN The Magazine on Twitter, @ESPNmag, and like us on Facebook.