Serena Williams surrendered the chance to make a third clay-court final of the season to protect her bid to land the surface's biggest prize at the French Open.
Williams apologized to her fans after she surprisingly withdrew from Saturday's Rome Masters semifinals.
Her voluntary withdrawal, citing a lower back problem, handed reigning French Open champion Li Na a direct passage into Sunday's final in the Italian capital.
Williams later confirmed the move was precautionary as she wanted to be "100 percent sure" for Roland Garros.
"I [felt] a little bit of ache and pain and don't want to go out there and start compensating and eventually hurt something else," she told reporters.
She later tweeted, "Everyone. I'm soooo sad and sorry I had to pull out of what has become my favorite tournament in Rome. Everyone has become so nice here in Rome and the tournament director is amazing. I will be here next year for sure."
Williams, whose solitary win at the French came a decade ago, has been in fine form on clay -- historically her weakest surface -- in the build-up to the season's second slam, which begins May 27.
The American's title successes in Charleston and Madrid saw her outclass the top-ranking players on tour, as she emerged again as the woman to beat heading to Paris.
