The Polish world No. 8 was stunned 3-6 6-7 by the lower-ranked Spanish player, who was more consistent, served better and made fewer mistakes.
Świątek, the 2020 French Open champion who turned 20 in May, is still scheduled to play mixed doubles with countryman Łukasz Kubot at the Olympic tennis tournament in the Japanese capital.
Iga Świątek. Photo: PAP/Leszek Szymański
Meanwhile, Poland's Hubert Hurkacz has advanced to the second round of the men's singles in Tokyo, with a 6-2 6-4 victory over Australia's Luke Saville on Sunday.
He will now play Liam Broady of Britain for a place in the last 16 on Tuesday.
Several other Polish tennis players have bowed out of the competition in Tokyo after losing their first-round matches.
Team Poland is still waiting for its first medal at the Summer Olympics in Japan.
Source: TVP Sport, Eurosport, PAP