Thiem has beaten Nadal four times on clay (4-7), but all best of 3 set matches. They have met here three times, including last year's final, Nadal has won all nine sets and Thiem has yet to even win five games in a set:
2018: 6-4, 6-3, 6-2 (final)
2017: 6-3. 6-4, 6-0 (semi)
2014: 6-2. 6-2, 6-3 (second round)
I think Nadal's alltime career record in five set clay court matches is 111-2 now, just the losses to Soderling and Djokovic here. Not only has he won the French 11 times, none have gone to five sets, 5 straight sets and 6 four sets, so 33-6 in sets overall in his 11 finals. That seems good.
All that being said, I would like Thiem's chances to at least push him some if it wasn't his fourth straight day playing, Nadal's quarter was Tuesday and his semi was Friday.