Tournament favourite Serena Williams withdrew from the Italian Open on Saturday with a lower back injury, the WTA said.

The American had been due to play China's Li Na in the first semi-final.

French Open holder Li will now go through to play either Maria Sharapova of Russia or Germany's Angelique Kerber.

Williams, who has won clay-court titles in Charleston and Madrid in the past month, was expected to give a news conference later on Saturday to explain the extent of the problem.