Del Potro, a former U.S. Open champion, will play Uruguay's Pablo Cuevas on Saturday. Cuevas defeated sixth-seeded Thomaz Bellucci of Brazil 6-4, 6-2 in this clay-court tournament leading to the French Open.
Germany's Kristina Barrois and Anabel Medina Garrigues of Spain, both unseeded, will meet in Saturday's women's final. Barrois reached her second career final with a 7-6 (2), 6-4 victory over Johanna Larsson of Sweden, and Medina Garrigues downed Romania's Monica Niculescu 6-1, 6-3. The Spaniard has won all nine of her career titles on clay.
Rain forced the suspension of the two other men's quarterfinals, with the matches to be completed Saturday morning before the semifinals.
Fourth-seeded Gilles Simon was leading fifth-seeded Milos Raonic 6-5 in the first set when play was halted. Seventh-seeded Kevin Anderson won the first set 7-6 (2) against second-seeded Fernando Verdasco.
Del Potro, seeded eighth, is fighting his way back up the rankings after missing much of last season because of wrist surgery. The Argentine also defeated Soderling last month, a straight-sets victory on a hard court in Miami.
This time, del Potro made a jittery start but broke Soderling in the fifth game and then used two aces to serve out the first set at love. The Swede came out stronger in the second set, breaking in the second game and opening a 4-1 lead. But the rain delay slowed his momentum and del Potro regained control.
Soderling, a two-time French Open finalist, last week lost his opening match on clay at Barcelona, where he was seeded third.