The Polish world number two and five-time Grand Slam winner could not force a single break point and dropped serve five times as she fell in 64 minutes.
Swiatek, usually excellent on clay, has struggled during her Madrid title defence.
She dropped sets against Alexandra Eala and Diana Shnaider, and then was dealt a bagel by Madison Keys in just 24 minutes on Wednesday.
However, the 23-year-old Pole bounced back strongly to defeat the American, but came undone against Keys’ compatriot Gauff, who beat Swiatek for the first time on the red dirt.
With this latest loss, Swiatek has only the upcoming Italian Open to help her get back to winning ways before she defends her French Open title later in May – her 2024 triumph at Roland Garros was the last trophy the former world number one lifted.
Gauff, 21, broke in the third, fifth and seventh games to blow Swiatek away in the first set, converting her second set point.
The world number four secured two more breaks in the second set and wrapped up victory at the first time of asking, on serve to love.
Gauff produced seven aces to Swiatek’s one in a dominant display.
“I felt great out here on the court today, much different to my first match here in the first round,” said Gauff, who had edged past Dayana Yastremska in three sets at the Manolo Santana stadium.
Gauff won her next three matches in straight sets at the Arantxa Sanchez stadium, before returning to centre court.
“I think the previous times we played on clay I don’t think I had a win against her yet,” Gauff said. “Obviously on clay she’s a different player, but clay is one of my better surfaces too, and I thought if I can beat her on hard I can beat her on clay.”
Gauff beat Swiatek at the 2025 United Cup and in the WTA Finals last year, both on hardcourt.
Last year’s runner-up Aryna Sabalenka faces in-form Elina Svitolina in the second semi-final later Thursday.
In the men’s draw Casper Ruud reached the semi-finals with a 6-3, 7-5 triumph over Daniil Medvedev.
The Norwegian ousted Taylor Fritz to reach the quarter-finals and secured his first win against the Russian in their fourth meeting.
Strong on clay, Ruud only converted two of his eight break points but it was enough to secure victory, with Medvedev, who dislikes the surface, not producing a single break.
Ruud put heavy pressure on Medvedev’s serve in the first set, breaking in the fourth game and serving it out.
The second set featured just a single break point, earned and converted by Ruud in the 11th game, enabling him to serve for the match.
Medvedev made 33 unforced errors and committed four double faults on a disappointing afternoon for the world number 10.
Ruud, ranked 15th, will face Francisco Cerundolo in the final four after he edged past Jakub Mensik 3-6, 7-6 (7/5), 6-2.
Fifth seed Jack Draper faces Matteo Arnaldi and Lorenzo Musetti takes on Gabriel Diallo in the other quarter-finals. (AFP)
