Moya, who is seeded sixth, will face 2004 champ Guillermo Canas in Saturday's semifinals. The other match features Viktor Troicki, a qualifier from Serbia and first-time ATP finalist, and unseeded Andrei Pavel of Romania.
Moya won this tournament in 1996, 2001, 2002 and 2003 and has a 41-8 record in Umag.
Ferrer had beaten Moya in their two most recent matches, including at Indian Wells in March. Moya had won all three of their previous meetings on Umag's clay and prevailed again, the former No. 1 coming from behind in both sets. Ferrer led 5-3 with a set point in the first set and 3-1 in the second but couldn't finish off Moya.
Fifth-seeded Canas of Argentina beat Gilles Simon of France 6-2, 6-1. Troicki, who hadn't won on the ATP this year until this week, followed his win over top-seeded Novak Djokovic by defeating Argentina's Carlos Berlocq 7-5, 6-2.
"I feel fantastic," Troicki said.
Pavel ousted seventh-seeded Italian Filippo Volandri 1-6, 6-2, 6-3. Volandri was hoping for a third final in Umag -- after 2003 and 2004 -- and dominated the first set. But Pavel started controlling Volandri with his serve and volley and didn't allow a break point in the second and third sets.