Serena pulls out in Rome with back injury
The withdrawal comes eight days before the French Open, but Williams said the move was mainly precautionary.
Rome: Serena Williams withdrew from the Italian Open with a lower back injury shortly before she was scheduled to meet Roland Garros champion Li Na in the semi-finals 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.
"I am in good form, I have a few days to get better. This is a good week to get better and I'm confident that I'll be 100 percent," she said.
The decision ends Williams' winning streak at 17 matches. She had won consecutive titles in Charleston, South Carolina, and Madrid, plus two matches in Fed Cup.
Besides Williams, 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.
Top-ranked Victoria Azarenka withdrew with a right shoulder injury before her third-round match. Also, Wimbledon champion Petra Kvitova was bothered by an abdominal injury in a three-set loss to Kerber in the quarter-finals.
And Italy's Flavia Pennetta retired with a right wrist injury while trailing Williams 4-0 in the first set on Friday. Williams played only four games in that previous round.
"Yesterday I might have had to retire as well," Williams said. "I don't know because we only played a bit and I didn't have time to check my condition."
Williams said she needed to be careful not to over-play, having been out of the game for a year in 2010 and 2011 because of a blood clot on one of her lungs.