The 62-minute victory earned the top-seeded Williams her 41st career singles title and second in two weeks. She won in Dubai last weekend, where she became the only active player to win 40 titles.
Overall, Williams has won four of her last five tournaments, including last year’s WTA Championships, and 20 of her last 21 matches.
In the men’s final, Nicolas Almagro of Spain defended his Mexican Open title, beating second-seeded Gael Monfills of France 6-4, 6-4.
Fourth-seeded Almagro, ranked 21st in the world, earned his fifth career title for beating the 10th-ranked Monfils.
Making her debut on Acapulco’s clay, Williams won three consecutive three-set matches then lost only six games in her last two matches in extending her winning streak to 10.
“During the week I had very hard games, but luckily I produced my best game for my toughest opponent,” Williams said.
She broke the second-seeded Pennetta four times in the final and saved the only break point she faced.
“I served very well, and I knew if do then it’s difficult to break me,” Williams said. “So I just tried to hold my serve and wait for her to make mistakes.”
Pennetta was playing in her sixth successive final in Acapulco, but has won only in 2005 and 2008. Her tournament-best record dropped to 28-5.
“I hope to return next year and try and reach a seventh final,” Pennetta said.
In the men’s final later, Nicolas Almagro of Spain was defending his title against second-seeded Gael Monfils of France.
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