Del Potro next plays Swede Robin Soderling, who beat American qualifier John Isner, 6-4, 6-4. Soderling had 11 aces during the match.
Second-seeded David Ferrer and American Sam Querrey, the No. 6 seed, will clash in the other semifinal.
Ferrer advanced when his quarterfinal opponent, Philipp Kohlschreiber of Germany, pulled out because of a shoulder injury.
Querrey was a 6-3, 6-2 winner over Nicolas Almagro of Spain, the No. 4 seed.