Gasquet, seeded second and the highest-ranked player at No. 15, overcame another Argentine, Agustin Calleri, 6-7 (3), 6-2, 6-3.
The Frenchman led 5-2 in the first set but allowed Calleri to come back and win the tiebreaker. Struggling with a sore right shoulder, Calleri had no challenge left when he again fell behind 5-2 in the second.
Gasquet advanced to his first final of the year, after making only one quarterfinal earlier in the season. In his only previous match against del Potro, Gasquet won in two sets at last year's Masters event in Indian Wells.
Del Potro rode his good serve to beat Schwank, who was in his first career ATP semifinal.
"Incredible, I can hardly believe it,'' Del Potro said after beating his third straight seeded opponent. "It would be great to take the title back home.''
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