World number two Halep outclassed Kristina Mladenovic 6-3, 6-1 in the northern French city of Rouen where 1,000 Romanian fans drowned out the larger home support. Garcia's forehand helped her get to 15-30 at match game, but Halep would not be denied, and blasted through the final three points of the tilt to close out a tremendous victory.
Victory was a diplomatic coup for France coach Julien Benneteau who teamed up Garcia and Mladenovic for the first time in Fed Cup since 2016 after which the pair had a bitter falling out which led to a self-imposed Fed Cup exile for Garcia, her country's top player.
"I knew it would be more hard than yesterday", the reigning Roland Garros champion said. But the same day Romania's Mihaela Buzarnescu lost the match against Caroline Garcia and the day ended with the score 1-1.
"I found out I was playing last night", said Parmentier.
Halep came into her clash with Mladenovic as favourite and despatched her French opponent 6-3, 6-1 to get Romania off to the ideal start.
Halep's win over Garcia, her sixth in seven meetings, took nearly three hours to complete with the crucial break in the final set coming in the eighth game when the French number one served up back-to-back double faults.
In the other semi-final in Brisbane, Australia and Belarus also shared the spoils with world number nine Ashleigh Barty sweeping past Victoria Azarenka after Aryna Sabalenka had ground down veteran Samantha Stosur.
Halep had earlier given the Romanians a 2-1 lead as she needed all her skill and determination to battle past aggressive baseliner Garcia 6-7 (6) 6-3 6-4 in a thrilling encounter that lasted two hours and 21 minutes.
The tie was sent into the crucial doubles match after two-time Australian Open champion Azarenka crushed Stosur 6-1, 6-1 in the reverse singles, following Barty sweeping past Sabalenka 6-2, 6-2.
"And then from Sam I could not have asked for more". 'I'm really happy with the way I played in the second set, Halep added.