Because he is the owner of the last three Grand Slams dating back to Wimbledon, and all of the wins have come against Nadal.
The Djoker has been very calm and collected, while Rafa has turned into a hothead of sorts on the court. That all changed on Monday, when Nadal used his patented spin on returns to really get under Djokovic’s skin. After one error, he drilled his racket against the net post and received a warning for his actions.
Since the passing of his grandfather, he has seemed mentally weak at times on the court, and it has led to 41 unforced errors.
When speaking with reporters after the match, he sounded like he was ready to regain his focus entering the second Grand Slam of the season (via ESPN):
I am going and preparing for a Grand Slam as I prepare for any other big event, with more focus. It's two weeks long and the most important tournament, and we always want to deliver out best game. I feel good on the court, and I need to make a few adjustments before Paris, but I'll be in top form.
With the second Grand Slam of the year on the horizon, I expect that to change quickly. I’m picking Djokovic to take home the title at Rolland Garros and retain his title as the best player in the world.
Nobody has a better combination of speed and power, and nobody has the stamina right now in the sport. With Rafa’s history of injuries in the past, will his body be able to hold up after playing multiple tournaments in a row?
Nadal has recaptured the No. 2 world ranking from Roger Federer, and there is no doubt he is more than deserving of the accolade, but right now Djokovic still has his number in Grand Slams of late.
Winning lesser events on clay is one thing, but a Grand Slam is a completely different animal. Sure, Nadal loves to play at Rolland Garros, but until somebody is able to unseat Djokovic in a major, you can’t bet against him.