American Venus Williams lost 5-7 6-2 6-3 to Serb title holder Jelena Jankovic in the Italian Open quarter-finals 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 (pictured) 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.

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.

The Serb fourth seed came back strongly in the second set, wrapping it up with a forehand down the line into the corner on Venus's serve.

The American paid for too many unforced errors in the final set as Jankovic won five consecutive games on the way to clinching the match.

"I made a lot of opportunities for myself, but then I made a mistake," Venus said.

"I played an aggressive game. Maybe I went for a little too much. But that's my game. On clay I'm a little less aggressive, but in the end I'm still going to play moving forward."

After the Serb's fourth victory in seven encounters with Venus, she said: "I've beaten her many times. I know that I always have to give my best if I want to beat her.

"She's really strong and I wanted to attack her as much as possible, put pressure on her and do the right things."

Ninth seed Schnyder betrayed no nerves against Sharapova, who will become world number one next week after Belgian Justine Henin retired from tennis on Wednesday.

Russia's Australian Open champion had to show great fight to come back from 5-2 down and save three set points in the 10th game but lost the first set in a tiebreak.

Schnyder had Sharapova on the back foot at the start of the second set too but the Russian responded again and evened things up with a break in its last game before clinching a thrilling match in the final set with the help of some thumping winners.

Cornet will be playing in her second Tier 1 semi-final when she meets Chakvetadze, having lost to eventual winner Serena at the Family Circle Cup in Charleston last month.

Chakvetadze had an easy time in the first set against the 64th-ranked Pironkova, who stunned Serbian top seed Ana Ivanovic by beating her in the second round on Wednesday.

The Russian lost her concentration in the second set after Pironkova took time out for treatment on an apparent shoulder problem but stormed back in the decider despite several times needing attention herself on the same area of the body.

Follow LIVE coverage of the Italian Open this week on British Eurosport and Eurosport 2!

British Eurosport is available in the UK on Sky channel 410 and Virgin Media channel 521, Eurosport 2 is on Sky channel 411 and Virgin Media channel 525.

Reuters