Struff had upset No. 10-seeded David Goffin in the round of 32 and No. 8-seeded Stefanos Tsitsipas in the round of 16 en route to his second quarterfinal of the season.
Nadal, who fell to Fabio Fognini in Monte Carlo, is looking for his 12th—and fourth consecutive—Barcelona trophy. It's worth noting he's won the tournament each time he's reached the semifinals.
Next up for the 32-yer-old Spaniard is either No. 3-seeded Dominic Thiem or Guido Pella. It will be a big test for the Spaniard either way, as both players have given him trouble on his beloved surface. Last week in the Monte Carlo quarterfinal, Pella broke Nadal's serve several times in the opening set before falling to the world No. 2. As for Thiem, he's beaten Nadal three times on clay.