Bacsinszky, the 30th seed, who needed a bandage applied to her right thigh midway through the first set, looked the more likely victor at that point as she sought to celebrate her 28th birthday by surpassing her semifinal run of 2015.
Following six breaks of serve, the first set went down to a tiebreak.
"I m not ashamed to say that she played better".More news: Third London Bridge attacker named
She has just dropped her serve and is trailing unseeded Jelena Ostapenko 4-3 in the opening set of the French Open semifinals. Both players were trying for the first time in his career to reach the final of the tournament series Grand slam. "I think was a battle." said the surprise package.
- Ostapenko took the first set 7-6 (4).
Her win over Pliskova was an interesting contrast of styles, with Halep's angles and movement pitted against Pliskova's serving and booming groundstrokes.
The contest was tight, Ostapenko winning 106 points to Bacsinszky's 105, but even that did not spoil the mood of the Swiss, who was obviously disappointed but dejected. I won the first set and was very close to making the final and some thoughts were in my mind. I'm looking forward to playing it.
But Ostapenko's fearless approach paid off after a series of breaks to start the decider, the Latvian landing the crucial blow to pull 4-3 ahead before sealing victory in two hours and 25 minutes.More news: Millwall fan who tackled London terrorists a 'hero' - Carl Frampton
She is the first Latvian to reach a Slam final, going one step further than Ernests Gulbis in 2014 when a young Ostapenko was in his players' box as he beat Roger Federer in the quarter-finals in Paris.
All of her big hitting with a racket that has a white ribbon attached to the frame for good luck gave Garcia fits: The last French player in the tournament made an eye-catching 31 unforced errors, to Pliskova's 13. "I'm really happy, I love to be here", Ostapenko said after the crowd serenaded her with a chorus of "Happy Birthday".
She is also hoping to win a maiden tour-level title at a Slam - a feat lasted accomplished by Gustavo Kuerten at the French Open in 1997 on the same day Ostapenko was born. With the second set delicately poised, Ostapenko seemed to lose focus and a number of spurned opportunities let her opponent back in.
But just when the storm appeared to have blown over Ostapenko, who knocked over former runner-up Sam Stosur and former world number one Caroline Wozniacki en route to the semis, hit another purple patch to roar home from 3-3.
Ranked No 47 in the world, Ostapenko is the lowest-ranked grand slam finalist since Sharon Walsh at the 1979 Australian Open.More news: Van strikes pedestrians on London Bridge