The American beat world No 1 Martina Hingis 6-3, 7-5, and Halard-Decugis of France beat an error-prone Lindsay Davenport by the same margin in the semi-finals on Saturday.
"I've beaten her before so it's nothing new," Williams said after winning for the second time in five career meetings against Hingis. "I'm sure she could have played a little better."
Hingis said: "She played well from the beginning. She attacked every ball and was always a step ahead. She didn't miss anything at all. Her serve - I couldn't break it."
Williams blended an attacking game and potent serve to keep Hingis on her heels throughout the high-quality contest. Williams won the opening set on the strength of a single service break in the fourth game when she hit a cross-court approach winner on break point.
Williams continued to apply pressure in the second set. Level at 5-5, she produced the crucial service break when Hingis sprayed a backhand pass wide, leaving her serving for the match.
Hingis did not appear too disappointed in losing saying, "there's not much I could have done better, except wanting it more, probably."
In an understatement, the 16th-ranked Halard-Decugis said of her surprise defeat of Davenport, "It is a big win for me. She's a great player and I am happy to beat her."
Davenport, whose forehand was out of sorts said, "Obviously, she played well and you hate to say it, but I think I was awful today. It's nothing against her but I missed a ton of forehands."
The reigning Wimbledon and US Open champion broke service in the sixth game of the second set to go ahead 5-2 but could not close it out. At 6-5, Halard-Decugis hit a cross-court forehand pass that sealed the win.
The former world No 1, could not find her form, committing 32 unforced errors to 12 for her opponent in a 67-minute defeat. But Halard-Decugis blended an effective serve with a good groundstroke game to keep the twice champion on the run.
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