The No.5-seeded Cirstea was the first of the two to advance, coming out of the blocks a bit slowly - she trailed 3-1 first set - but eventually picking up steam and cruising past Olga Govortsova, 63 62.
"Before I came to Stanford I was practicing really, really hard for two weeks," Cirstea said. "It's nice to see all of this work paying off. I put my head down, did my daily things and worked hard, and now I'm feeling much better on the court and in my legs. I feel like a different player out there on the court.
"And I really love it here in Stanford. It's such a nice start to the US Open Series."
The No.3-seeded Cibulkova followed with a harder-fought win, regrouping after letting a 5-1 first set lead slip away and eventually closing out No.7 seed Urszula Radwanska in straight sets, 75 63.
"The key today was staying aggressive," Cibulkova said. "She's the kind of player who waits for your mistakes and uses your power, and I was doing great in the first set until 5-1. But then I started hoping she'd just give me the set, and all of a sudden it was 5-all and she was up 30-0 on my serve.
"But I made that game, and all of a sudden I started playing my game again and won the set. In the start of the second set I played more down-the-lines. She couldn't read where I was going anymore."
Cibulkova leads Cirstea in their head-to-head series, 2-1. The Slovak won their first two meetings handily, but Cirstea won their last meeting - it was in the quarterfinals here last year, in fact.
"The last time I played her was here last year," Cibulkova said. "She's another player who likes the fast surfaces, because she has a big serve and returns really well, so I'm prepared for a quick game tomorrow, and hopefully I'll get some experience from the two matches I've had so far this week."
"She's a really good player and is playing some really good tennis this week," Cirstea said. "If you're in the semis, especially at a big tournament like this, it means you're in good form and have played some good matches out there. It's not going to be an easy match tomorrow, but that's why I train hard, to get opportunities like this. I'll just give my best out there and fight and we'll see what happens."