It's all about Nadal's famous top spin. On dry surfaces, it can be deadly, But on damp surfaces, it naturally loses some of its luster. If you take away that weapon from Nadal's—or at least ease its affect—you open up the door for another contender to steal the trophy away from the 26-year-old.
World No. 2 Andy Murray—who withdrew from the tournament due to a back injury—said as much on Monday in an interview with ESPN.
Murray said, via SI.com:
I think if it heats up, if it’s warm, I take Rafa. If it’s sort of rainy and damp and cold then that changes things. It gives Novak, in my opinion, a much better chance of winning.
Nadal himself was also cognizant of the weather, noting the "only negative thing is this cold," via ABC Grandstand Sport.
Back in August 2011, the New York Times came out with a video that highlighted Nadal's forehand. It showed just how much topspin Nadal can generate (measured at an average of 3,200 rpm).
A look at the video displays the genius behind Nadal's forehand. Thus, you can imagine how much of a relief it could be for his opponents when that topspin is deadened by the weather. It can be a game-changer when you aren't facing shots that dive away or in front of you at such velocity.
Murray has a point, and Nadal understands the concept as well. That's not to say Nadal still doesn't have incredible topspin in colder weather, but there's no question he loses some of his edge in colder conditions.
You combine the cold weather with Djokovic's ability to handle Nadal's forehand and you can see why some believe the Serb has a shot at capturing his first career French Open title. Plus, while Nadal has a 12-3 career record against Djokovic on clay, Djokovic did defeat Nadal in Monte Carlo this year.
One thing's for sure: By the end of the 2013 French Open, Nadal and Djokovic will have a pretty good idea of what the weather is like at Roland Garros. The final outcome of the tournament may be enough of an indicator.