"Definitely disappointed with how today went. But I guess I wanted to give it a try. I never really knew how bad it was until I kind of went out there," Raducanu told reporters.
"I'm still learning when it's right to push my body and push through it, and when it's not. I guess that's something I'm learning at these tournaments.
"After Madrid I thought that maybe taking one, two days off, it would just go away because a lot of the other small niggles I've had, they've kind of gone away after taking two days off."
American qualifiers Madison Brengle and Lauren Davis both advanced in straight sets in the event seen as a final warm-up for the French Open. Brengle beat fellow qualifier Marta Kostyuk 6-4, 6-3 and Davis defeated 2017 French Open champion Jelena Ostapenko 6-2, 6-3.
The Associated Press and Reuters contributed to this report.