Rezai claimed her third WTA Tour crown, rallying to turn back a Williams challenge in the second set by saving six set points. She then moved quickly to finish off the former world No. 1 and notch the biggest win of her career.
Rezai, ranked 24th in the world, also defeated two other former top-ranked stars in Madrid: Justine Henin in the first round and Jelena Jankovic in the quarterfinals.
Williams fell short in her bid to collect her third title of the year, falling to 2-2 in finals. She was able to defend her Dubai and Acapulco championships but lost to Kim Clijsters in the Miami final.
With the victory, Rezai pocketed a first prize of $780,580 and boosted her career record against Williams to 2-1.