But world number one Serena Williams pulled out of the tournament because of an injury to her left knee.
WTA Tour official Darrell Fry said that, after Williams was examined in Los Angeles, her doctor told her that she needed to rest.
The Wimbledon champion also pulled out of last week's Bank of the West Classic in Stanford, California with pain in her left knee.
Pierce was lethal against Razzano, controlling the centre of the court with her hard groundstrokes and dictating with her return of serve.
The injured-plagued Frenchwoman has not won a tournament since she triumphed at Roland Garros in 2000, but against Razzano she showed the form that once took her to number three in the world.
AGGRESSIVE PIERCE
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The 28-year-old, who now meets former Wimbledon champion Conchita Martinez of Spain, is ranked 73rd in the world and needed a wild card to get into this week's event.
Pierce, who also won the 1995 Australian Open, said she felt better than she had in three years. Last month, she reached the fourth round of Wimbledon.
"I have everything in order and am on the right path to get my conditioning back," she said. "My game is starting to come back and it's a great feeling.
"I haven't achieved everything that is possible for me in tennis yet and the best is yet to come."
Spain's Virginia Ruano Pascual beat Russia's Alina Jidkova 2-6 6-2 6-4 to earn a clash with second-seeded Belgian Kim Clijsters, who won the Bank of the West Classic title.
Greece's Eleni Daniilidou overcame Italy's Rita Grande 7-5 6-3 and will face third seed Justine Henin-Hardenne of Belgium on Tuesday.
