Pliskova came up with good serves when she needed them at 4-5, 15-40 in the second set to hold that game, and went on to dominate the tiebreak and third set thanks also to a barrage of unforced errors from Cirstea.
"I didn't expect to win," Pliskova said. "She was playing amazing in the first set and I didn't have any chance to play my game. She was playing fast and she was killing all my second serves. In the second set I was just trying to hold my serves and hoped that in the tiebreak anything could happen."
Pliskova's last appearance in a semifinal was in Kuala Lumpur last February, when she also went on to claim her first and only singles title.
She faces Julia Georges of Germany, who beat fifth-seeded Elena Vesina of Russia 5-7, 6-3, 6-4.
"I'm happy that I've won hard matches here," Pliskova said. "I played my last semifinal in Kuala Lumpur which is almost a year. So I'm happy that I'm not going to lose (ranking) points."
In another quarterfinal, Andrea Hlavackova of the Czech Republic upset eighth-seeded Peng Shuai of China 6-2, 7-5.
Hlavackova, a doubles specialist who has two Grand Slam doubles titles, will have to go through fourth-seeded Ekaterina Makarova of Russia or Japanese veteran Kimiko Date-Krumm to reach her second singles final.