Sharapova, who missed the 2016 tournament because of a doping ban and a year ago after she was refused a wild card, was broken 6 times, committed 27 unforced errors and won just 5 points on her own serve in the second set.
Halep, the runner-up in 2014 and 2017, will face Spanish third seed Garbine Muguruza for a place in Saturday's final.
The top seed roared back in the second set, breaking serve in the opening game and going on to take it 6-3.
In Thursday's other semi-final, US Open champion Sloane Stephens will take on fellow American and close friend Madison Keys who she beat 6-3, 6-0 in the final in NY last September. "When I'm talking about progression or looking back at the weeks, I think there are a lot of good things, like the way my body has handled the long matches that I have played, the back-to-backs".
The No. 3-seeded Muguruza had lost all three previous meetings against Sharapova, who won two of her five Grand Slam titles at Roland Garros.
The two met in the semifinals earlier this year at the Australian Open, with Halep prevailing 9-7 in a third set there. Muguruza sensed weakness and pounced for a 4-0 lead.
Sharapova didn't need to play a point in the fourth round, because Serena Williams pulled out of their much-anticipated match with an injured chest muscle. Muguruza essentially got that round off, too: Her opponent, Lesia Tsurenko, retired with an injury after only two games.
Muguruza will face Simona Halep or Angelique Kerber in the semifinals.
Both Keys and Stephens completed their Grand Slam quarterfinal set in Paris this year and Keys has now made the semifinals or better at 3 of the 4 majors.
The second quarter-final between Marin Cilic and Juan Martin del Potro is also finely balanced with the players tied at 5-5 in the tiebreak in the opening set. That might very well have been a relief to 10-time champion Rafael Nadal, who was not at his best on Wednesday (Thursday NZT) and dropped a French Open set for the first time since 2015.
In the other women's quarterfinal, No. 1 Simona Halep takes on former No. 1 Angelique Kerber in a rematch of their epic Australian Open semifinal in January. Halep has won 12 of her last 14 matches on clay court.
The top seed is a two-time runner-up at Roland Garros but was forced to dig deep against the German, who is enjoying a fine recovery year after a hard 2017, as Kerber stormed into the lead.