The world number one fell at the first hurdle at Arianke Tennis Park, going down 6-4 6-3 against her Spanish opponent.
“It was a tough day. A disappointing day. I can’t lie about that,” Barty said. “I can’t hide behind the fact that I wanted to do really well here. Today wasn’t my day.
“I knew I wanted to try to take the match on and it would be a fine line of not pushing too hard and not getting in the patterns I didn’t want to get stuck in. (I was) Too erratic.”
There were no such problems for Naomi Osaka; the home favourite overcame China's Zheng Saisai in straight sets.
Crowned Wimbledon champion this month, Barty was unable to build on that momentum; amassing a whopping 55 errors.
World number 48 Sorribes Tormo took full advantage to set up a second-round showdown with France's Fiona Ferro.