That is the opinion of BBC Sport tennis reporter David Law.
Nadal is the strong favourite to land his record 11th French Open title this afternoon.
Thiem has won three of his nine meetings with Nadal, but is yet to beat the Spaniard in a best of five sets match.
Law was asked by the Tennis Podcast co-presenter Catherine Whitaker: “If they play the final 10 times, how many times does Thiem win it?”
He said Thiem could win if Rafael Nadal has a bad day, but does not think that is likely.
“One,” he replied.
“As things stand I would say one, really.
“I do feel that Nadal is a very heavy favourite.
“Thiem has got to find a way to stay with him early on.
“But, I say ‘we know Nadal is going to be awesome’ and I certainly expect him to be awesome, (but) he might just have a bad day.
“It’s possible, I suppose. I’d be very surprised.”