The world number one succumbed to a 6-1 1-6 6-4 defeat to the fifth seed, who ended the French Open champion's 18-match winning streak on the dirt.
It was Garcia's first win over a top-ranked player and sent her into a semi-final against Jasmine Paolini after the Italian came from behind to beat Viktorija Golubic 1-6 6-2 6-2.
Swiatek said: "Maybe I wasn't as disciplined as in previous tournaments, but I'm happy. I felt like my brain didn't intuitively predict where the ball would go, but I'm still learning, right?
"The first two sets depended on her – whether or not she would be in control. At the end of the second she lost control and took advantage.
"In the third set, she got back to her game. But if I had a better first serve percentage, maybe it would have been different.
"I don't know what decision I will make next year. Maybe longer preparation [on clay] will be necessarily, or maybe I'll give up this tournament. We'll see."
Kateryna Baindl became the first lucky loser to reach a WTA Tour semi-final this year by overcoming Petra Martic 1-6 6-3 6-4. Next up for Baindl is Ana Bogdan, who beat Laura Pigossi 6-1 6-1.
World number two Anett Kontaveit was also on the receiving end of an upset, going down 6-1 6-1 to Anastasia Potapova in 49 minutes at the Prague Open.
Potapova will take on qualifier Wang Qiang for a place in the final, with Czech pair Marie Bouzkova and Linda Noskova – only the second female player born after 2004 to reach the last four of a WTA Tour tournament – in action on the other side of the draw.
Defeated in the final of the Hamburg European Open last weekend, Kontaveit got straight back to winning ways in rapid time with a 6-0 6-1 win over Ekaterine Gorgodze.
The world number two got 81.8 per cent of her first serves in, with the overwhelmed Gorgodze only able to win nine returning points across the entire match.
Up next for Kontaveit is Lucie Havlickova after the Czech defeated compatriot Barbora Palicova 6-2 7-6 (7-5).
Fourth seed Cirstea was sent packing from the first round by 19-year-old qualifier Oksana Selekhmeteva, whose 7-5 6-4 success was her first in the main draw of a WTA Tour event.
Dominika Salkova marked her WTA main-draw debut with a 6-1 6-4 win over Ylena In-Albon, while Barbora Krejcikova delighted the home crowd with her 6-3 6-1 triumph against Anna Blinkova.
Due to persistent rain, only three matches were completed at the Warsaw Open on Tuesday.
Clara Burel, Ana Bogdan and Jasmine Paolini all advanced to the second round with wins over Anna-Karolina Schmiedlova, Nuria Parrizas-Diaz and Danka Kovinic respectively.
Mertens, who reached the fourth round at Wimbledon at the start of July, succumbed to a 6-3 2-6 6-2 defeat to Magda Linette – her first loss to the Pole after three straight-set victories in their previous encounters.
The Belgian got just 52 per cent of her first serves in and won 38.5 per cent of points behind her second serve while only converting one of the five break points she earned.
Sixth seed Alison Van Uytvanck retired in the first set of her match against Dalila Jakupovic, while eighth seed Marie Bouzkova overcame Sinja Kraus 6-2 7-6 (7-2).
In Warsaw, fifth seed Garcia by no means had it easy in a 7-5 6-4 success over Doi.
The Frenchwoman committed eight double faults and only got 55 per cent of her first serves in, but by saving six of the eight break points she faced she was able to get the job done in straight sets.
Polish wildcard Maja Chwalinska was a 7-5 6-2 victor over qualifier Rebeka Masarova in only her second Tour-level tournament.
