American fifth seed Serena Williams withdrew from her Italian Open quarter-final with French qualifier Alize Cornet because of a back injury.

Williams said the back injury should not affect her bid to win the French Open.

"It just happened all of a sudden in practice," the eight-times Grand Slam winner said. "I just went for a shot and then my back got a little stuck.

"I don't expect this to cause any problems with my preparation for the French (Open). It just happened all of a sudden. I feel like I'm going to be good going into Roland Garros."

Williams said she waited until the last minute before pulling out of the match with 18-year-old French qualifier Cornet, who was handed a walkover to a last-four meeting with Russian sixth seed Anna Chakvetadze.

The American was optimistic about winning back the French title she claimed in 2002.

"I'm just going to keep training for Roland Garros and get ready for that," she said. "I'm in really good form, so I'm going to be going in there with a couple wins under my belt."

Cornet, who on Thursday knocked out last year's finalist Svetlana Kuznetsova of Russia, earns a walkover to the last four, where she will meet Anna Chakvetadze, a 6-2 3-6 6-1 winner over Bulgarian qualifier Tsvetana Pironkova.

Russian sixth seed Chakvetadze had an easy time in the first set against the 64th-ranked player, who stunned Serbian top seed Ana Ivanovic in the second round on Wednesday.

The Russian lost her concentration in the second set after Pironkova took time out to have treatment on an apparent shoulder problem.

She then stormed back in the decider, despite needing attention herself several times on the same area of the body.

Reuters