The 10th-seeded Williams, the last American remaining in the men’s or women’s singles draw, was upset by the 30th-seeded Bacsinszky 5-7, 6-2, 6-1 in a 2-hour, 12-minute fourth-round outing on Sunday.
Last year, Bacsinszky turned back Williams in the same round in straight sets.
The 36-year-old Williams rebounded from a 1-5 deficit in the first set, but then ran out of energy to fight back Bacsinszky in the final two sets.
This was a record-setting 20th time that Williams was playing at the French Open. She reached the final here once, in 2002, when she fell to sister, Serena, in the championship match.
Williams’ departure means that the women’s draw has no former Grand Slam champions remaining. The last four Grand Slam champions —Svetlana Kuznetsova, Garbine Muguruza, Samantha Stosur and Williams — all lost Sunday.