The Barcelona natives, the first Spaniards to qualify for the season-ender since Emilio Sanchez and Sergio Casal in 1994, held their nerve to win a third set championship tiebreak 10-3 after the first two sets were split 7-5 3-6.
The only other Spanish duo to win the title were Juan Gisbert and Manuel Orantes in 1975 in Stockholm when the title was decided by a round-robin group.
Granollers and Lopez, who beat world No.1 pairing Bob and Mike Bryan in group play last Monday, will be part of Spain's Davis Cup final team against the Czech Republic later this week.
