Both players were relatively unfancied prior to the event having had patchy years prior to the French Open, but have been able to reproduce their previous high levels to get to this stage.
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, who has lost all three of her Grand Slam finals - including the Australian Open, to Caroline Wozniacki, in January - arrived in Paris on a hot streak, reaching at least the quarter-finals in her last nine claycourt tournaments and losing only one set on her way to facing Kerber in Paris. "If I do that, I will be okay after the match, no matter the result".
She pointed her right index finger at her temple when she eventually completed a 6-7 (2), 6-3, 6-2 comeback.
"She loves clay. She loves Roland Garros".
Muguruza has not dropped a set so far in this year's tournament as she seeks a second championship in Paris and third major trophy overall.
Halep spent an hour longer on court than Muguruza in their respective quarter-finals on Wednesday, needing three sets to beat German 12th seed Angelique Kerber. She was helped by her opponent as the Russian produced three double-faults in the opening game to gift it to the Venezuelan.
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Muguruza raced to a 4-0 lead and never looked back.
Maria Sharapova left the French Open without much of a fight, but had enough left in the tank to deliver a parting shot at Serena Williams. Muguruza essentially got that round off, too: Her opponent, Lesia Tsurenko, quit with an injury after only two games.
He was also helped by a couple of poor errors by Nadal at key moments in the match and broke the Spaniard twice to lead 3-2 before play was suspended briefly due to rain. Sharapova was the French Open champion in 2012 and 2014; Muguruza took the trophy in 2016.
The rain seemed to help settle the defending champion as he came back out and broke straight back, before breaking again to put the second set in sight.