The British number one is having by far her best season on clay and reached the final of the WTA Tour event in Rabat last season before dashing to the Spanish capital and defeating Alison Riske in round one.
The heavy schedule looked like it might well have caught up with her as, after playing a fine opening set that she came close to winning, Konta fell away dramatically to go down 7-5 6-1.
Konta has only ever beaten one top-10 player on clay but she played a superb first six games and had a chance to lead 5-2.
She could not take it and Halep, the champion here in 2016 and 2017, fought back to level before forcing two set points at 5-4. Konta saved those but then missed more opportunities to lead 6-5 and Halep eventually converted her fifth set point.
The world number three put her foot down at the start of the second set and Konta won just two points in the first four games.
She dug in to at least get on the board but could not stall Halep's momentum as the Romanian clinched victory in an hour and 28 minutes.