Pliskova of the Czech Republic beat No. 28 seed Caroline Garcia of France 7-6 (3), 6-4 in the quarterfinals, while Halep of Romania put together an epic comeback to oust No. 5 seed Elina Svitolina of Ukraine 3-6, 7-6 (6), 6-0.
Halep found herself down a set and 5-1 -- and even had to save match point -- but she pulled off the escape act in spectacular fashion.
Pliskova reached the U.S. Open final last year but had never been past the second round in five previous appearances in Paris.
Pliskova and Halep will meet for a spot in the final with the WTA's No. 1 ranking possibly on the line.
After current No. 1 Angelique Kerber was ousted in the first round, both Halep and Pliskova have a shot at the top ranking, with Pliskova needing to reach the final to achieve it and Halep needing to win the title.
Halep had not dropped a set all tournament long, but she ran up against her first real test in the quarters against a familiar opponent. Svitolina defeated Halep in a dramatic Rome final just over two weeks ago.