The 31-year-old Williams blitzed the world number 42 Slovak 6-1, 6-1 in a quarter-final match that took just one hour on Friday.
"She had nothing to lose," Williams said of Rybarikova. "I knew I had to play really, really good today."
Williams next faces third-seeded Agnieszka Radwanska of Poland on Saturday for a spot in the final.
Radwanska powered her way past Sara Errani of Italy 7-6 (7/1), 7-5 in an earlier quarter-final match.
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It was a powerful display of tennis from the top seed Williams, who hammered seven aces and broke Rybarikova a half dozen times.
"I'm definitely happier with the return game today," Williams said. "I could still use some improvement. She served pretty well, but I was pretty happy with the results from today."
Williams is 8-0 since her fourth-round loss at Wimbledon to Sabine Lisicki.
"I just felt like I was too tight, and just put a lot of pressure on myself there," she said of the loss at Wimbledon. "I just needed to relax more than anything, which is what I'm doing."
"It might come as a surprise for many people," said Cirstea of her upset win. "It's not really a surprise for me, because I know how hard I have been working for the past couple of weeks. "It's nice to see the reward.