World number one and defending champion Iga Swiatek took her Stuttgart winning streak to 10 matches on Friday with a straight sets quarter-final victory over Emma Raducanu while title rivals Aryna Sabalenka and Coco Gauff suffered shock defeats.
Top seed Swiatek, the 2022 and 2023 champion at the German clay-court tournament, saw off former US Open champion Raducanu 7-6 (7/2), 6-3.
Reigning Wimbledon champion Marketa Vondrousova beat world number two and three-time Stuttgart runner-up Sabalenka 3-6, 6-3, 7-5.
US Open champion Gauff, the world number three, fell 3-6, 6-4, 7-6 (8/6) to Ukraine's Marta Kostyuk.
The 22-year-old Swiatek needed just over two hours to defeat Raducanu who was playing in her first quarter-final since 2022.
"She started playing at the beginning pretty loose, like she had nothing to lose, and I totally get that. Sometimes it is like that," said Swiatek.
"But I knew I was kind of questioning if she's going to be able to keep the same intensity throughout the whole match. It wasn't about service games or return games. I was just waiting for my chances to break back and I was sure that I'm going to get them."
Despite her loss, there were encouraging signs of a return to form for Raducanu.
Her two wins for Britain in the Billie Jean King Cup win against France last weekend followed by two victories in Stuttgart mark the first time she has won four matches in a row since her 2021 US Open title triumph.
The former world number 10 came into Stuttgart ranked at 303. Twelve months ago, Stuttgart proved to be her final event of 2023 before undergoing surgery on both wrists and her ankle.
Swiatek will face fourth-seeded Elena Rybakina, who defeated Jasmine Paolini 6-3, 5-7, 6-3, for a place in the final.
Vondrousova reached her first semi-final since clinching the Wimbledon title last July when she stunned Sabalenka.
The sixth-seeded Czech celebrated her first win over Sabalenka in six years after losing four in a row between 2018 and 2023.
Australian Open champion Sabalenka had been attempting to make the Stuttgart final for a fourth successive year.
The powerful Belarusian crunched 35 winners to Vondrousova's 17, but committed 48 unforced errors compared to 19.
Vondrousova crucially converted seven of eight break points while Sabalenka only capitalised six times from 15 opportunities.
Vondrousova will face Kostyuk for a place in Sunday's final after the 27th-ranked Ukrainian avenged her Australian Open quarter-final loss to Gauff.
The 21-year-old Kostyuk had needed over three hours to defeat Laura Siegemund in the first round before saving five match points to get past Zheng Qinwen in the last 16.
On Friday, she battled back from 4-2 in the second set and 3-1 down in the third while Gauff had to save a total of seven match points.
The Ukrainian carved out 21 break points in all and hit 35 winners as she reached her third semi-final of 2024.
"Well, definitely means a lot, because to be consistent in tennis, it takes a lot of work and a lot of discipline," Kostyuk told wtatennis.com
"Everything around you has to work really well."
