Del Potro, who is also the tournament's defending champion, saved four break points in the first set. The Argentine is recovering from a wrist injury and it is not clear whether it may have been bothering him.
Earlier Friday, third-seeded Czech Tomas Berdych survived a difficult first set, including a tiebreak in which he had five set points. He regrouped and went on to win 6-7 (9), 6-2, 6-4 against Jerzy Janowicz of Poland. Berdych and Gulbis will meet in the semifinals.