Williams, who will retake the world No. 1 spot when the new rankings are released Monday, is 3-0 in the round-robin segment of the tournament, fueled by a six-game winning streak after the fifth-seeded Dementieva, a Russian Olympic gold medalist, took a 2-1 lead in the first set.
"It was definitely a match I had to be really focused for because she's a really good player," Williams said. "She's one of those late bloomers who tends to get better and better and better as every year goes on (and) I was probably playing the most fit girl on tour."
Third-seeded Svetlana Kuznetsova of Russia, this season's women's French Open winner, lost 6-2, 6-7 (3-7), 6-4 to Serena's older sister, Venus, the seventh-seed and defending champion.
Venus Williams rose to 1-2, while Kuznetsova sank to 0-2.