Statistics are tea leaves at best so far in the season, but give a general feel for the level of a players. Time to break down this matchup like I did for Thiem and PCB (both predicted Tsitsi in 2
@cc0509 
). How do the players games match up.
Tsitsipas strengths:
1. Has goated against high spinners Gasquet and Thiem this year so he should play extremely well against Nadal.
2. First serve. Had three aces in a row against ARV and saw some 226 kph serves against PCB (140 mph)

3. Forehand - can hit through anyone on tour including Nadal, may be hard to read
4. Return - great reach should help against Nadal first serve and won 36% first serves against Shapo on clay who has a hooky lefty serve as well
5. Backhand - a raking spinny, deep shot that seems to go deeper under pressure. Against righty backhands its been highly effective. One of the keys to taking to Nadal is having the ability to attack his forehand and keep him honest. The Tsitsipas backhand is ultra reliable so it may defy Nadal and another poster pointed out that Nadal has a worse record against one-handed backhands despite his infamous abuse of the Federer backhand.
6. Going from defense to offense - surprising depth when really stretched allows Tsitsipas to often take advantage of the next shot and suprise the opponent with a winner.
7. According to some posters Tsitsi has great point construction like Kuerten. To me Stefanos plays like Federer in his dreams on clay. He's a very, very different style of clay courter from Nadal or Thiem.
8. Tough under pressure - very easy for Tsitsi to fall back on a grinding game plus he always has #6 above.
Tsistsipas weaknesses:
1. Take time away from him on both sides and he becomes very ordinary. PCB and ARV failed at this task (their forte) because they couldn't keep it up without UEs creeping in due to Tsitsipas heaviness and depth. On clay its difficult to expose Tsitsipas in this manner, but Nadal was very aggressive with his court position against Goffin today so he'll have a shot at this, but his heavy spin will actually slow his balls through the air and see Tsitsi strength #1 above (goats against spinny players).
2. Lefties - you could see it with ARV's lefty forehand doing some damage to the Tsistsipas backhand. The great question for this match is will Nadal be able to hit winners off the Tsitsi backhand. Trading cross court shots alone may not work. Tsitsipas is a very crafty player on clay so he may be able to exploit Nadal's leftiness to counteract this seeming mismatch.
3. Stamina - was forced into some grinding rallies late in the match with Busta that left him in some kind of Del Potro Del Possum mode (perhaps even more dangerous than the Del Possum). Needless to say this may become an issue for RG and would be in a three setter with Nadal.
This could be the recipe for an amazing upset.