ROME (AP) - Serena Williams withdrew from the Italian Open with a lower back injury shortly before she was scheduled to meet French Open champion Li Na in the semifinals on Saturday.
Williams said the problem also affected her earlier this week.
"It was feeling a little stiff and I don't think this is the right moment to force (it)," she said. "I just want to relax and get ready for the next few months."
The withdrawal comes eight days before the French Open, but Williams said the move was mainly precautionary and that she'll be ready for the clay-court major.
"This is a good week to get better and I'm confident that I'll be 100 percent," she said.
The decision ended Williams' winning streak at 17 matches. She had won consecutive titles in Charleston, South Carolina, and Madrid, plus two matches in Fed Cup.
In the Rome final, Li will face either defending champion Maria Sharapova or 11th-seeded Angelique Kerber of Germany, who were playing later.
The men's semifinals feature five-time Rome champion Rafael Nadal against fellow Spaniard David Ferrer and top-ranked Novak Djokovic vs. Roger Federer.
Several other players are also nursing injuries.
Fourth-ranked Andy Murray skipped last week's Madrid Open with a back problem and said it was still affecting him after a three-set loss to Richard Gasquet in the third round this week.
Wimbledon champion Petra Kvitova was bothered by an abdominal injury in a three-set loss to Kerber in the quarterfinals. And Italy's Flavia Pennetta retired with a right wrist injury while trailing Williams 4-0 in the first set Friday.