Hard to see any value just because it's possible. I can give you a rundown on the numbers based on Nadal. While Thiem has been the 2nd best clay player
he would be in his 1st slam final and so if Nadal doesn't make it hes still playing someone like Del Potro who in this scenario just defeated Nadal. Upsets happen all the time but i'd be wanting bigger odds than 5 surely?
Nadal in F.O Finals
05 - Def. M. Puerta 6–7(6–8), 6–3, 6–1, 7–5 [4 sets]
06 - Def. Fed: 1–6, 6–1, 6–4, 7–6(7–4) [4 sets]
07 - Def. Fed: 6–3, 4–6, 6–3, 6–4 [4 sets]
08 - Def. Fed: 6–1, 6–3, 6–0 [3 sets] [Won Tournament without losing a set]
10 - Def. Soderling: 6–4, 6–2, 6–4 [3 sets] [Won Tournament without losing a set]
11 - Def. Fed: 7–5, 7–6(7–3), 5–7, 6–1 [4 sets]
12 - Def. Djoko: 6–4, 6–3, 2–6, 7–5 [4 sets]
13 - Def. Ferrer: 6–3, 6–2, 6–3 [3 sets]
14 - Def. Djoko: 3–6, 7–5, 6–2, 6–4 [4 sets]
17 - Def. Stan: 6–2, 6–3, 6–1 [3 sets] [Won Tournament without losing a set]
French Open final Stats.
Played 10; Won 10; 40% 3-0 60% 3-1
French Open Stats:
83W - 2L: 97.64% Won
Thiem 2017 F.O before Nadal
Sets Lost - 0
Last Result - d. Djokovic 7-6 6-3 6-0
V Nadal at F.o Lost 3-6 4-6 0-6
Thiem v Nadal H2H Clay
2017
Barca: Nadal 6-4 6-1
Madrid: Nadal 7-6 6-4
Rome: Thiem 6-4 6-3
FO: Nadal 6-3 6-4 6-0
2018
Monaco: Nadal 6-0 6-2
Madrid: Thiem 7-5 6-3