"It just happened all of a sudden in practice," Williams said before heading to a hospital for tests. "I just went for a shot and then my back got a little stuck."
Williams did not appear in much pain, carrying a large pocketbook over her left shoulder as she entered a news conference immediately after her withdrawal.
"I didn't feel anything the last match, and I don't expect this to cause any problems with my preparation for the French," she said. "It just happened all of a sudden. I feel like I'm going to be good going into Roland Garros."
This tournament is an important clay-court tuneup for the French Open, which begins May 25.
Jankovic beat Venus Williams 5-7, 6-2, 6-3, with the American committing 57 unforced errors to the Serb's 24.
"I had a lot of great opportunities, and I made a lot of opportunities for myself. But then I made a mistake," Venus said.
Jankovic's semifinal opponent will be either second-seeded Maria Sharapova or 2005 runner-up Patty Schnyder, who were playing the night match.
Cornet will meet sixth-seeded Anna Chakvetadze, who held off Bulgarian qualifier Tsvetana Pironkova 6-2, 3-6, 6-1.
Jankovic captured last year's title at the Foro Italico without dropping a set, recording the best week of her career. She improved to 4-3 in her career against Venus Williams, who dropped to 12-6 this year.
Serena Williams has had a splendid start to the year, compiling a 23-2 record and winning three titles. Her withdrawal deprived the tournament of a chance at the first all-Williams final since Serena beat Venus for the 2003 Wimbledon title.
