The top-seeded Nadal broke Struff's serve in the final game of each set to stay on course for a record-extending 12th title on the outdoor clay courts at Barcelona. He next faces the third-seeded Thiem, who beat Guido Pella 7-5, 6-2.
"I am getting better and better as the tournament goes on," Nadal said. "These are two consecutive weeks that I will at least have reached semifinals on clay." Nadal lost in the semifinals in Monte Carlo last week and then needed three sets to get past Leonard Mayer in his opening match in Barcelona. He saw off David Ferrer in straight sets on Thursday.
Struff, who beat last year's runner-up Stefanos Tsitsipas in the previous round, fought back from an early break to make a match of it. Nadal avoided a first-set tiebreaker by pushing Struff into a 0-40 hole on his final service game. He closed out the set when Struff could only stab his passing shot off the court.
Nadal then finished the match by chasing down a ball and landing a backhand return just inside the opposing corner, improving his record in Barcelona to 61-3. Nadal is 8-3 against Thiem, including a win in the 2017 final in Barcelona. But the Austrian is the only player to have beaten him on clay in each of the past two seasons.
Nadal said that Thiem "is a candidate to win any tournament on clay." While Nadal has won 57 clay-court titles, eight of Thiem's 12 titles have come on the surface. Kei Nishikori, who won here in 2014 and 2015, also advanced to the final four after beating Roberto Carballes 6-4, 7-5.
The fourth-seeded Nishikori will play Daniil Medvedev, after the Russian hit five aces in defeating Nicolas Jarry 6-3, 6-4.
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