Defending champion Elina Svitolina will play Belinda Bencic in the other semi-final after she capped off an unbeaten round robin campaign with a 7-5, 7-6 (12/10) victory over late replacement Sofia Kenin.
Pliskova believed she would be the underdog against Barty. "She's having a great year, so it's going to be super tough," she said.
"It's going to be a big challenge but I have nothing to lose."
An aggressive Pliskova started red hot against Halep winning 13 of the first 16 points en route to blitzing through the first set in just 20 minutes.
Halep appeared rejuvenated after receiving a pep talk from coach Darren Cahill between sets as the match flipped.
The Wimbledon champion finally hit the scoreboard by breaking to open the second set and she rediscovered her touch, hitting menacing slices to rattle Pliskova.
Halep broke Pliskova's suddenly tentative serve three times to force a decider and her momentum continued in the third set to jump out to a 2-0 lead.
The twists continued with Pliskova regaining control but she was unable to serve it out in the ninth game.
Halep's late rally was short-lived as Pliskova booked her third straight semi-finals appearance at the WTA Finals.
The Romanian lamented her inconsistent performance. "I didn't play my best tennis," Halep said. "But she was good. It's a tough loss."
In the earlier match, Svitolina was pushed in the dead-rubber by world No.12 Kenin, who replaced Bianca Andreescu after the US Open champion suffered a knee injury against Pliskova on Wednesday.