I think there's a chance. Djokovic, as well as he's been playing, is still susceptible to a great player playing well. He does not play as aggressively as consistently as he used to because his basic game wins him almost every match he plays. But when Nadal is on form on clay or when Wawrinka zones out on clay, he is vulnerable. 

I agree we don't know enough about Nadal's form yet. He plays Fognini, his kryptonite a year ago, in the Barcelona QF, and that could be interesting. Nadal won Monte Carlo without serving well and without hitting his forehand nearly as well as he did in his prime, but he fought like he used to and played big points like he used to. On clay, it's all in his head. Even with his diminished game, there's no reason he can't be the second best player in the world on clay if his confidence is high. 

 

The loss to Siegemund doesn't look as bad, given that she made it to the final.  Still, Halep is not going to be my pick to win Roland Garros.  At this point, I am leaning towards Kerber (deep in Charleston, won Stuttgart).  It'll be a much easier pick if Azarenka is on the other side of the draw; she could win, but she's only won one clay singles title in her career, back in 2011.

As for Serena, well, all things are possible with Serena.  It wouldn't be shocking to see her win.  It also wouldn't be shocking to see her lose early.

Perhaps Halep needs that sinus surgery that she's delayed.

Let's see if Djokovic rebounds in Madrid or Rome.  Wouldn't be surprised if he lost to Federer in Madrid (if he plays, though Federer has traditionally played well there), but a loss to anyone else would be bad news for his Roland Garros plans.