The two clay-court stars are set to go head-to-head this afternoon with a semi-final spot the reward for the winner.
Nadal beat Diego Schwartzman yesterday to take his consecutive sets wins on the dirt to 50.
The last player to beat the world No 1 on clay was Thiem at the Italian Open 12 months ago.
And the Austrian is hoping for similar success this time around, although he is doubtful after being hammered 6-0, 6-2 at the Monte Carlo Masters last month.
“It’s the biggest challenge in tennis right now. It’s really, really tough,” Thiem said.
“I’m going into the match to win, even though it’s not very realistic, the way he plays now, the way I play now.
“(Rome last year) was one of the best matches of my life. If I want to repeat… it takes the same.
“I have to play very aggressive with not many mistakes.”
Meanwhile, Nadal has hailed Thiem as one of the best players in the world.
“Of course, he is one of the best players of the world because the ranking speaks for itself,” Nadal said.
“But on this surface, he’s one of the most dangerous players you can face, one of the ones that have a lot of possibilities to succeed.
“Tomorrow is a key match in the tournament.
“It’s one of the matches where I have to be a step ahead.”
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