The sixth seed already has a title on clay to her name after winning in Stuttgart and she ruthlessly dumped the favourite out of the Caja Magica 6-4 6-3 in an hour and nine minutes. "Let's not talk about that any more and let's continue on what we have ahead which is what I am concerned about".
Nothing worked well for Simona and she failed to mix up her game and force Karolina to play from more awkward positions on the court, cracking under the pressure of barrage fire from the other side of the court.
"I never really thought I can beat her, but this time somehow today I felt like I will have for sure chances".
There were two more breaks in the second set, the latter confirming Pliskova's comfortable victory as a tired Halep backhand hit the net.
Mertens had won every match on clay this season ahead of her showdown with the world number one, claiming titles in Lugano and Rabat.
The two went toe-to-toe as the advantage swung back and forth, but Puig paid the price for not getting enough first serves across the net and failing to take her opportunities when they arose, allowing the Czech to prevail. "I feel fantastic since in the last six matches I lost to her".
But five-time Grand Slam champion Maria Sharapova missed out on what would have been only her second semi-final this year as the Netherlands' Kiki Bertens came from behind to win their quarter-final 4-6 6-2 6-3.
Pliskova's only prior victory from her previous seven matches against Halep was in the 2016 Fed Cup.
Breaking to go 5-4 up, Suarez Navarro spurned four match points after leading 40-0 before taking a tumble on the final point but Svitolina sent her return long and the Spaniard burst into tears in front of the raucous home supporters.
In other match yesterday, Austrian fifth seed Dominic Thiem rallied from one set down to beat Croatia's Borna Coric 2-6, 7-6 (5), 6-4, while Serbian qualifier Dusan Lajovic shocked Argentine fourth seed Juan Martín Del Potro 3-6, 6-4, 7-6 (6).
Pliskova, a former world No.1, is scheduled to play against either her countrywoman Petra Kvitova or Russia's Daria Kasatkina for a place in the final.
France's Caroline Garcia also made the semi-finals with a straight sets defeat of Spain's Carla Suarez Navarro.