The top-seeded Medina Garrigues was aiming for her fourth Strasbourg title after also winning the clay-court event in 2005.
Third-seeded Peng Shuai of China reached the quarterfinals after beating Pauline Parmentier of France 6-3, 7-6 (6).
Earlier, Medina Garrigues said she felt a sudden pang at the end of a long rally.
"After the final stroke, I felt a violent pinch in my lower back,'' she said. "It is a strong pain which stopped me from bending down, straightening back up, basically from moving around.''
Despite the setback, Medina Garrigues hopes "two or three days rest'' will be enough for her to recover in time for the French Open, which begins Sunday at Roland Garros.
Aravane Rezai of France and Ayumi Morita of Japan both reached the last eight.
Rezai advanced 6-4, 6-1 against Anastasiya Yakimova of Belarus, while Morita beat French player Julie Coin 4-6, 6-0, 6-4.
In other matches, Lucie Hradecka of the Czech Republic beat Alla Kudryavtseva of Russia 7-5, 6-0; Monica Niculescu of Romania topped Yuliya Beygelzimer of Ukraine 5-7, 6-0, 6-4, and Viktoriya Kutuzova of Ukraine downed French player Stephanie Foretz 3-6, 6-3, 6-2.
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