The Spaniard was beaten 7-5 4-6 6-4 by Chilean Nicolas Jarry, who will face second seed Alexander Zverev in the second round.
Elsewhere, Fernando Verdasco beat fellow Spaniard Feliciano Lopez 6-4 6-3 on the Rafa Nadal Court.
Thiem stuns Nadal to reach Barcelona final
Meanwhile, Argentina’s Diego Schwartzman came back from a set down to see off Japan’s Yoshihito Nishioka 4-6 6-4 6-2.
Leonardo Mayer beat big-serving Romanian Marius Copil 6-3 6-7(3) 7-5 to set up a second round meeting with 11-times champion and top seed Rafael Nadal.