After losing the first game to Andrea Petkovic of Germany in Grand Slam singles play, and trailing 30-0 in the second, Williams stretched to her right to return a serve, hit the ball and then yelped in pain, clutching toward her stomach.
"Obviously I just couldn't play," Williams said. "I couldn't move, it was too painful."
Afterward, Williams said the injury was in her "psoas" muscle, which flexes the hip and spinal column.
"The last 48 hours, I just did as much pain management as I could," she said. "A lot of times when you play, you get that adrenalin and that blocks pain. I just didn't get enough of that today. I was hoping for some magic that I could recover."
Williams' younger sister, defending champion Serena Williams, didn't even make it to the tourney because of a foot injury.