From 2003 to 2007, Roger Federer dominated men’s tennis. He dominated tennis to an extent where it became an upset if he entered a tournament and didn’t actually win it. In 2006, he won 81 matches and lost just 4; it was almost 82-3, until he got injured in the World Tour Final against Nalbandian, going down in 5 sets. This was his era, and players like Andy Roddick, Lleyton Hewitt and Marat Safin would not forget this. 

Rafael Nadal dominated men’s tennis completely in 2008. He again dominated the scene in 2010. For these two years, he was unbeatable. But his dominance occurred almost every alternate year; he couldn’t keep it up for more than 365 days at a time. Therefore, there was no such thing as the Rafael Nadal era. Federer was always around even now, even though the Swiss player came up second best. Nadal’s rise also coincided with the gradual rise of Novak Djokovic and Andy Murray. There was also Juan Martin Del Potro, who annihilated Nadal at his peak in the US Open semifinal of 2009. Nadal was a dominant force, but his stats and Grand Slam wins meant that his body often came in the way of a sustained period of ruling the tennis world. 
Then came Novak Djokovic. With Nadal at his peak, and Federer always around his peak, even Andy Murray had begun to discover his best days. But the Serbian joker, who often attracted the ire of fellow professionals because of his frequent retirements and silly sideshows, began to hit his stride after he eliminated gluten from his diet. He catapulted himself into the limelight immediately, and this was during an era that had two of the greatest men’s tennis players of all time playing together. 
Djokovic, over the last four years, has managed to carve out his own rightful place in history. The last two years have been the Novak Djokovic era; despite the presence of at least three other top players, Djokovic has reached a stage where he is expected (and mostly does) win every tournament he enters. He is gracious, fiercely competitive and has worn down rivals like Nadal and Federer over time. He has endured heartbreak in Paris more than once, just like Federer did for many years against Nadal, but he has come back stronger to win Wimbledon or the US Open. In 2011, he won 3 of the 4 Grand Slams in the year. This was dominance, but it only lasted till 2012—where he lost to Nadal in Paris, Federer in London, and Murray at New York. He was still at his peak, reaching perhaps the final 4 of every tournament, but just didn’t finish things off the way he used to. 
In 2013, Rafael Nadal returned to his best form, and got back some of the losses against Djokovic. But the Serb didn’t drift away. Again, he won the Australian Open, before losing the French to Nadal, Wimbledon to Murray and the US Open to Nadal again. He lost the 2014 Australian Open to Wawrinka, the French to Nadal again, but defeated Federer to win Wimbledon, before losing to US to Nishikori. In 2013 and 2014, with Nadal, Murray and Federer playing well, Djokovic won 2 Grand Slam titles. He still won things. He won more masters titles than anyone else, and just like Serena endured a lean 2014 as far as Grand Slams went, Djokovic cashed in on the other tournaments. 
In 2015, Djokovic was expected to sweep all 4 Slams. He will probably win the US Open, but his loss to Wawrinka at the French Open will hurt him a lot. It is admirable how he bounced back to defeat an inspired Federer at Wimbledon despite not playing his best tennis. 
Djokovic, quite simply, is in beast mode, in an era peppered with greats and future greats, and has managed to make this time his own. 
Irrespective of his Slam titles and the numbers (that often don’t do him justice), he could go down as the best player of his era. He has won more Grand Slam matches than Nadal already, and has been far more consistent at the top. He will probably finish behind Federer in one column—the number of consecutive weeks at World No. 1. Federer had 236, and unless Murray does a Nadal, Djokovic will remain at the top for a while. Another year or two at the very least. 
The Serbian tennis champion should be proud. And he is only 28. Though Federer stopped winning Grand Slams at 29, Djokovic looks like he is only getting started. 17 Grand Slam finals is no joke; 9 titles isn’t either.