Stuttgart - Germany's former world No 1 Angelique Kerber had to quit with a right thigh injury on Thursday at the Stuttgart Grand Prix and was swiftly followed by second seeded Wimbledon champion Garbine Muguruza who called it quits with back pain.
The limping Kerber handed over a 6-0, 2-0 second-round win to Anett Kontaveit of Estonia.
Barely an hour later, Spain's Muguruza was unable to go on due to lower back pain after dropping the opening set of her first match of the week 7-5 to Russian Anastasia Pavlyuchenkova.
In all, three of the day's seven matches ended in retirements -- earlier third seeded Elina Svitolina of Ukraine was 2-6, 6-1, 3-2 ahead of Marketa Vondrousova when the Czech qualifier quit with right thigh trouble.
Muguruza came to Stuttgart on the back of a seven-match win streak through her Monterrey title and two Fed Cup wins.
The pullouts leaves a path for top seed Simona Halep at the event which also lost three-time winner Maria Sharapova in the opening round on Tuesday.
Kerber, a former two-time major winner now ranked 12th, said she had been feeling the problem for several days, even as she beat Petra Kvitova a day earlier.
"I had a lot of pain, especially this morning. I tried to go out there and play for the home crowd, I thought I could," she said.
"In the end it was too much, it was bad in the early games. I think I made a bad movement as well.
"Now we have to see what's going on, I have to make a decision on Madrid (starting May 5)."
Kerber, who was treated off-court after going down 0-5 in 17 minutes, was playing with her right thigh strapped.
Kontaveit, the world number 31, swept the opening set and then broke to start the second before Kerber gave up a game later after 39 minutes on the clock.
The former number one last retired in Madrid 2017 against Eugenie Bouchard in the third round.
"It's hard on clay for me, it's not my natural movement," Kerber said. "Things were much worse for me today."
Roland Garros champion Jelena Ostapenko continued to build for her Paris title defence with a 6-3, 6-0 win over Zarina Diyas.
The Latvian was joined in the quarter-finals at the Porsche arena by fifth seed Karolina Pliskova as the Czech beat Russian qualifier Veronika Kudermetova 7-6 (7/4), 6-3.
Ostapenko, ranked fifth on the WTA rankings, broke her 52nd Kazakh opponent five times as she won in just 63 minutes.
"The early matches are tough but I think I played really well today and I'm just happy with the win," Ostapenko said.
The 20-year-old added that said she is still adjusting to life as a Grand Slam champion.
"I expected I could win a Grand Slam but not at this early age. I just understood a couple of months later."
American CoCo Vandeweghe extended her unexpected run of clay form, beating defending champion Laura Siegemund of Germany 6-4, 4-6, 6-3.
The Californian, ranked 16th after breaking into the top 10 last season, said her secret appears to be playing relaxed.
"I probably have more fun on clay since I don't care too much about it," she said prior to only the second clay quarter-final of her career where she will face Halep.
Carolina Garcia, a first-round winner over Sharapova, knocked out 15-year-old Ukrainian prodigy Marta Kostyuk 6-1, 3-6, 7-5.
Results from the WTA Stuttgart Grand Prix on Thursday (x denotes seeding):
Coco Vandeweghe (USA) bt Laura Siegemund (GER) 6-4, 4-6, 6-3
Elina Svitolina (UKR x3) bt Markéta Vondroušová (CZE) 2-6, 6-1, 3-2 - retired
Caroline Garcia (FRA x6) bt Marta Kostyuk (UKR) 6-1, 2-6, 7-5
Karolina Pliskova (CZE x5) bt Veronika Kudermetova (RUS) 7-6 (7/4), 6-3
Jelena Ostapenko (LAT x4) bt Zarina Diyas (KAZ) 6-3, 6-0
Anett Kontaveit (EST) bt Angelique Kerber (GER) 6-0, 2-0 - retired
Anastasia Pavlyuchenkova (RUS) bt Garbiñe Muguruza (ESP x2) 7-5 - retired