After failing to win a game in the opening set, and trailing in both the second and third, the Russian fought back strongly to overcome the world number two in one hour 43 minutes.
Third seed Davydenko, who defeated the world's top player Roger Federer in the semi-final, produced 52 winners against the second-seeded Spaniard's 37 to claim the trophy.
American Andy Roddick recovered from a second set slump to win the Brisbane International title with a 7-6, 7-6 victory over defending champion Radek Stepanek of the Czech Republic on Sunday.
Dementieva wins
French Open champion Svetlana Kuznetsova and her fellow Russian Elena Dementieva won their opening round matches at the Sydney International on Sunday. Kuznetsova beat Alisa Kleybanova 6-2, 6-7, 7-5, while Dementieva defeated Italy's Francesca Schiavone 6-2, 7-5.
Slovakia's Daniela Hantuchova eased out Virginie Razzano of France 6-3, 6-0 in less than an hour to advance to a second round clash with Dementieva. Women's world number one Serena Williams has a first-round bye and was scheduled to meet Justine Henin in the second round before the Belgian pulled out with a leg injury sustained during her loss to Kim Clijsters in Saturday's Brisbane final.
