Thiem, the runner-up to the Spaniard in the 2017 Madrid final, was the last player to defeat Nadal on clay. That was at the 2017 Italian Open in Rome - 357 days ago
Since then Nadal went on to claim the French Open for the 10th time while racking up a record streak of 50 consecutive sets. It was a sequence that also furnished the 31-year-old Spaniard with titles in 2018 in Monte Carlo and Barcelona.
"A player like Dominic has a lot of potential. Whenever he plays really well, it's very difficult to stop him," admitted Nadal.
"He played well and I didn't play well. We're not playing a game where the differences are big or massive. The differences are very small."
The result also means that Roger Federer will reclaim the world number one ranking on Monday.
On Saturday, the pressure will be on the fifth seeded Austrian. Thiem has lost all six of his matches against 31-year-old Anderson but none of their encounters have been on clay nor have they followed such a significant coup for the 24-year-old.
"It takes a really good match to beat Rafa on clay,” said Thiem. “But I think a very important thing also was that I went in with the attitude that I can beat him.
"Two weeks ago in Monte Carlo, he killed me 6-0 6-2. It was very important I went in with a positive attitude, with an attitude to win. I should go into every match like this against him."
Anderson hit 15 aces and 35 winners as he powered past the Serbian qualifier Dusan Lajovic 7-6 3-6 6-3. It will be the South African’s first appearance at a Masters event which is considered just below the four Grand Slam tournaments in Melbourne, Paris, London and New York in terms of prestige.
The unseeded Canadia Denis Shapovalov will play second seed Alex Zverev from Germany in the other semi-final.