Unseeded Sevastova was playing her first semi-final at premier mandatory level, the grade immediately below grand slam tournaments.
She went toe to toe with her higher-ranked opponent early in the match before Halep broke in the fifth and the seventh games and then comfortably served out the first set.
The defending champion got off on the wrong foot in the second, and trailed 3-0 after surrendering her first service game.
But some words of advice from coach Darren Cahill prompted a swift turnaround and she won the next six games to march into her third Madrid final in 73 minutes.
Halep finished runner-up to Maria Sharapova in 2014.
(Reporting by Richard Martin; editing by John Stonestreet)
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