The story of Novak Djokovic's surge

It's tough to rest on your laurels in men's tennis these days. No sooner does one legend establish himself than a challenger begins nipping at his heels. When Roger Federer was piling up Grand Slam victories and making his case for being the greatest of all time, there was Rafael Nadal, who kept resisting him on clay and slowly made inroads on grass and hard courts until, with his U.S. Open victory last year, the Spaniard finally stood alone at the top.

Then just when it looked like the era of Nadal had well and truly arrived, along came Novak Djokovic. The year began with talk about Nadal completing the "Rafa Slam" with a win in Australia, but it was Djokovic who ended up taking the title, also following up with wins at the Indian Wells and Miami Masters. Nadal was expected to reassert himself once the clay season arrived, but the Serb has once again blocked his way, defeating the King of Clay in back-to-back finals in Madrid and Rome over the past two weeks. Who is the favorite for the French Open? It is the first time since Nadal's French Open debut in 2005 that the question has had to be asked.

Nadal has won the title five times in the past six years, but it is Djokovic who comes into the tournament on one of the most impressive winning streaks of the Open era. He is currently 37-0 for the season -- five wins behind John McEnroe's spectacular 42-0 start in 1984, but already better in quality if not quantity. McEnroe's wins were notched mostly at smaller indoor events, with the biggest result a loss in the French Open final. Djokovic has secured a Grand Slam and four Masters titles in his seven tournament wins so far. McEnroe beat (still Slamless) Ivan Lendl five times, Jimmy Connors and Mats Wilander. Djokovic has beaten Federer three times and Nadal four times, also getting Andy Murray twice and securing a win over David Ferrer on clay.

Each successive win over Nadal, in particular, has elevated Djokovic's standing to new levels. First came back-to-back wins over the world No. 1 in hard-court finals at Indian Wells and Miami, and the second of those three-set victories was a striking sight: the Serb with once-suspect stamina tiring out the normally indefatigable Nadal on a hot, muggy Florida day. Then came two straight-sets wins over Nadal on clay, where Djokovic had lost all 10 previous meetings. The win in Madrid was surprising, but could be explained away -- the altitude gives the ball more speed, the clay is too slippery for Nadal's liking and the relatively cramped conditions prevent him from showing the full extent of his impenetrable defense. The win in Rome was staggering -- these were the traditional clay-court conditions in which Nadal is at his toughest. And while the Spaniard had been affected by a fever at the beginning of the tournament, Djokovic had gone three brutalizing hours with Murray in the semifinals the day before. But the Serb still managed to hold his own in long, punishing exchanges and found more winners and big serves at the big moments.

It was a major shake-up. Djokovic must now be considered a real threat to Nadal's supremacy at the French, adding intrigue to an event that in recent years seemed to be turning into a one-man show.

Where did it all come from? "A lot of people have been guessing and speculating what the secret formula of my good form was but there is no secret," Djokovic said recently. "It's just that all the pieces have fallen into place after years of hard work and we are now reaping the rewards."

With his seamless, pinball-machine tennis, Djokovic has long been seen as a potential force at the top of the game, but in the past struggled to sustain success. Physically, allergies and breathing problems wore him down and prompted frequent retirements. Mentally, he found it difficult to recover from defeats and ignore off-court challenges. Technically, he struggled with equipment changes and hitches in his serve.

Now, his body, mind and game all seem to be working in sync, and he has been moving and hitting with machine-like precision.

The place to begin is with the streak itself. The full length of the streak is 39-0, with two of those wins coming late last season. And that's where the story of Djokovic's surge begins. They came at the end of last season, when he led the Serbian team to its first victory in the Davis Cup. After winning a pressure-packed final in front of the home fans, the players shaved their heads on court and celebrated hard for "two days and nights" afterward. Most players who manage to navigate the unpredictable four rounds and 20 months of a successful Davis Cup campaign count it as their biggest achievement or close to it, and Djokovic is no different.

"The Davis Cup final gave me a lot of confidence," he reflected in Australia. "To be able to win with a team for your nation was much different than anything I have experienced personally on the tennis court."

A little confidence boost can work wonders. Djokovic's serve, which had plagued him for most of 2010, also magically restored itself by the time he got Down Under. "I was aware of what I [was doing] wrong. But once it gets into your head, it's really hard to get it out of your habit," he said. "It's great and encouraging fact that I can rely on my serve in the important moments -- that gives you a little bit of the relief and advantage."

His early-season success was not a surprise in itself. After all, Djokovic had shown himself capable of Grand Slam brilliance before, winning the Australian Open in 2008. This time, however, he has been able to maintain that form, without any physical or mental letdown. Last month, returning home to play his family-owned event in Belgrade, he revealed a little more about just how he has done so.

Ten months ago, Djokovic hired doctor and nutritionist Igor Cetojevic, who worked to figure out the allergies and heat problems that had so frequently left him gasping for breath during matches. "We're just waiting for DHL to give us a canister of oxygen that I will put next to me on the changeovers," Djokovic joked after one bad spell.

Now, as Miami showed, he is outlasting the likes of Nadal in three-set matches. Djokovic points to Cetojevic as key in the improvement. "He's done a great job in changing my diet after we established I am allergic to some food ingredients like gluten," the Serb told reporters in Belgrade. "It means I can't eat stuff like pizza, pasta and bread. I have lost some weight, but it's only helped me because my movement is much sharper now and I feel great physically."

He has also maintained his focus in tournament after tournament, early rounds and big matches alike. Living up to his nickname, the "Djoker" remains outgoing and emotive, but says he can now channel that energy rather than struggle to suppress it. "My mindset is different now," he said. "I'm more stable emotionally. Even though I do show my emotions during the match, you can see, I do feel tougher mentally. I think that's a process of learning and experience."

All together, it has been enough to meet every challenge Djokovic has faced so far this season. But is it enough to surmount one of the toughest challenges ever presented in tennis -- defeating Nadal on clay in a best-of-five-set match? It has been done only once, by a firing Robin Soderling at the French two years ago, but even that victory came against the backdrop of Nadal's hurting knees and family troubles. Still, it has been done, at least, and if it is to be done this year, there is no doubt Djokovic has established himself as the most likely to do it.

A Djokovic win over Nadal in Paris would have much the same effect as Nadal's wins over Federer at Wimbledon in 2008 and Australia in 2009, turning the challenger into a conqueror. A standoff is hardly guaranteed, of course. For Djokovic, the emotion of his victories and the pressure of the streak will continue to build as the tournament goes on. Nadal, meanwhile, has not been looking as aggressive or invulnerable as last season, when the Spaniard swept through the clay undefeated. But thanks to Djokovic's wins over Nadal in Madrid and Rome, winning the French Open, even without defeating Nadal, would be enough to establish him as the new king, with the No. 1 ranking as formal acknowledgement.

But he has been quick to play down any projections after his win in Rome. "He's the king of clay," said Djokovic of Nadal. "It's just two tournaments, whereas he's been so dominant for so many years."

"In the last few months, he's been on a different level to the rest," Nadal said. Then he added, "Being a big champion is not only about being able to win every week but when you are able to wait until the right moment."

For Djokovic so far this season, there have been no wrong moments.

Kamakshi Tandon is a freelance tennis writer for ESPN.com.