Published: May 19, 2012 12:06 PM EDT
Updated: May 20, 2012 6:31 AM EDT

ROME (AP) - Serena Williams has withdrawn from the Italian Open with a lower back injury shortly before she was scheduled to meet French Open champion Li Na in the semifinals on Saturday.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.