UPDATE 4-Tennis-Williams sisters out of Italian Open

(updates after Sharapova win)

By Paul Virgo

ROME, May 16 (Reuters) - American Venus Williams lost 5-7 6-2 6-3 to Serb title holder Jelena Jankovic in the Italian Open quarter-finals on Friday while her sister Serena pulled out at the same stage of the tournament with a back injury.

Jankovic's semi-final opponent will be Russia's number two seed Maria Sharapova who beat Swiss Patty Schnyder 6-7 7-5 6-2.

Fifth seed Serena said her withdrawal from her match with 18-year-old French qualifier Alize Cornet should not affect her preparations for the French Open, which starts on May 25.

Cornet knocked out last year's finalist Svetlana Kuznetsova on Thursday and her walkover sets up a semi-final with another Russian, sixth seed Anna Chakvetadze, a 6-2 3-6 6-1 winner over Bulgarian qualifier Tsvetana Pironkova on Friday.

Neither seventh seed Venus nor Jankovic were able to stamp their authority on the early phases of their quarter-final, but the American took two of the three first-set games that went with serve to get the upper hand.

UNFORCED ERORS