Serbia's Novak Djokovic returns the ball during a training session at Roland Garros stadium in Paris. (Christophe Ena/Associated Press)

Federer, Nadal. Nadal, Federer. For the past several years, the rivalry between these two players has dominated men’s tennis, and the only question among fans was, which one was Batman to the other’s Robin.

However, this is looking more and more like the year of the “Djoker.”

Novak Djokovic, once a better bet to perform a spot-on impression of Federer or Nadal than to actually pull off a victory over either man, is now on the verge of scaling heights unknown to even that extremely accomplished duo.

In fact, no one in any sport is having a better 2011 thus far than Djokovic: He is undefeated in 37 matches, and if he reaches the French Open final, he will become the first player in more than seven years not named Roger or Rafael to attain the No. 1 ranking.

Perhaps more significantly, a trip to the final would give Djokovic the ATP Tour record for wins to start a season, breaking John McEnroe’s 1984 mark of 42. Winning the tournament would tie him with Guillermo Vilas for most consecutive victories, 46, in the Open era. (Djokovic won twice to close out 2010.)

“Well, it’s no secret that it’s been the best five months of my career. I’m on a great streak and I definitely want to keep on going,” Djokovic said at a news conference earlier this month.

Not bad for someone whose quick and entertaining rise to the top of the men’s game seemed destined to stall at No. 3, behind two players considered among the best of all time.

Djokovic, born in Belgrade, Serbia, in 1987, first appeared on the ATP Tour in 2004, and he finished 2005 as the youngest player in the top 100. The next year he was the youngest in the top 20, and the year after that, the youngest in the top 10.

By this time, the gregarious Djokovic had earned a reputation as one of the tour’s foremost comedians — hence the nickname “Djoker” — most notably charming 2007 U.S. Open attendees with on-court impersonations of Nadal and Maria Sharapova. His take on McEnroe at the 2009 Open went over so well that the legend himself left the television booth and hit the court to give Djokovic a taste of his own medicine.

As YouTube clips of Djokovic’s antics began to go viral, however, targets such as Federer, Sharapova and Andy Roddick indicated that they weren’t always amused. Chastened by some of the criticism he was receiving, Djokovic eventually started toning down the “Djoker” act.

Another issue for the Serb was that his ambitions to reach the top of the men’s game were consistently thwarted. He was winning plenty of matches but had just one Grand Slam title, the 2008 Australian Open, to show for it. In addition, Djokovic, who finished the 2007, ’08, ’09 and ’10 seasons at No. 3, was only 6-13 against Federer and 7-16 against Nadal.

But two events occurred last year, one physical and one mental, that set the stage for Djokovic’s phenomenal 2011. Midway through 2010, he was discovered to have an allergy to gluten, a protein found in many grains. Djokovic changed his diet, avoiding pizza and pasta, and the efforts eventually translated into elite fitness.

Then, right at the end of the 2010 season, he won two matches to help Serbia take its first Davis Cup. Succeeding under enormous pressure to deliver for his countrymen meant the world to Djokovic, and he has credited that experience with giving him a major psychological boost. Armed with sky-high confidence, the Serb rolled to his second Grand Slam title, again at the Australian Open, a 3-0 record against Federer and took the latter’s No. 2 ranking.

“This year Novak is the player of the world, and he is the most improved,” said Mary Carillo, a Tennis Channel analyst. “I think the questions we always had about him were about his health and about his heart. He has answered these questions. He has just become a superlative tennis fighting machine.”

But Djokovic’s success against Nadal has been the most intriguing trend leading into the year’s second Slam event, the French Open. After all, the Spaniard is aiming to tie Bjorn Borg’s record of six titles at Roland Garros and is widely regarded as the finest clay-court player ever. Indeed, Djokovic had never beaten Nadal in nine tries on that surface.

However, any pessimism proved to be nothing more than pre-2011 thinking. Two weeks ago, Djokovic broke through against his nemesis, not only beating Nadal for a title on clay, but doing it in front of a highly partisan crowd in Madrid. He followed that up with a championship in Rome, again topping Nadal on a clay court without so much as losing a set.

“I have won against him twice . . . and he has given me a lot of confidence for the French Open,” Djokovic told members of the media in Rome. “But this is only a couple of tournaments this year and he has been dominant on this surface for so many years.”

Maybe so, but no one has been as dominant this year on any surface, in any sport, as Novak Djokovic. And quite possibly as soon as the hardware is handed out at Roland Garros, he will have tennis’s top ranking and a prominent piece of its history in his hands, as well. Then, for those who thought that the conversation about men’s tennis began and ended with Federer and Nadal, well, the joke will be on them.