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.”
“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.
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.
By John Solomon
How the government's punishing of the exposure of official wrongdoing can linger for years
Independent voices from the TWT Communities
A mother of three and a passionate conservative, Shirley Husar changes the game.
This column will cover anything that has anything remotely to do with the game of baseball, from the game itself to mid-summer trades to offseason moves.
Eye on Europe, the Middle East and Africa
Benghazi: The anatomy of a scandal
Vietnam Memorial adds four names
Cinco de Mayo on the Mall
NRA kicks off annual convention