Wimbledon champion Angelique Kerber said the pain from an ankle injury was "not my excuse" after her shock first-round exit at the French Open.
The German fifth seed lost 6-4 6-2 to world number 81 Anastasia Potapova.
The three-time Grand Slam champion said she "did not have much expectation" after pulling out of the Madrid and Rome clay tournaments this month.
Elsewhere, Spanish 19th seed Garbine Muguruza came from a set down to beat American Taylor Townsend 5-7 6-2 6-2.
Muguruza was broken for a second time late in the first set by the world number 83 but breezed through to win the next two sets in the first main draw match on the new Simonne Mathieu court.
The 2016 French Open champion will play Sweden's world number 172 Johanna Larsson in the second round, following her victory over Slovak Magdalena Rybarikova.
Russian Potapova, playing in her first French Open, faces Marketa Vondrousova of the Czech Republic next.
"My phone is exploding at the moment," said a delighted Potapova after her surprise win.
Meanwhile, Czech second seed Karolina Pliskova eased into the next round with a straight-set victory over Madison Brengle.
The 27-year-old, who reached the semi-finals at Roland Garros in 2017, won 6-2 6-3 to set up a meeting with Slovakian qualifier Kristina Kucova.
Last year's runner-up Sloane Stephens also progressed on Sunday, with the 26-year-old American winning a tie-break in the second set to beat Japan's Misaki Doi 6-3 7-6 (7-4).