Rome, Italy –  Last week's Madrid titlist Serena Williams and her big sister Venus were among those recording third-round victories Thursday at the $2.168 million Italian Open tennis event, a clay-court French Open tune-up.

The ninth-seeded former No. 1 Serena held her serve throughout in whipping Spaniard Anabel Medina Garrigues 6-3, 6-1 in 76 minutes, while the wild card Venus upset fifth-seeded Aussie Samantha Stosur 6-4, 6-3 at Foro Italico. Venus and Stosur combined for 10 service breaks, including six for the American victor.

The former top-ranked Venus was the Rome champion back in 1999 and runner-up in 1998.

Stosur, who stunned Serena in last year's U.S. Open finale, was last year's Italian Open runner-up to Maria Sharapova.

The 2002 Rome champion Serena, now 16-0 on clay this year, upset current world No. 1 Victoria Azarenka in last week's finale on the controversial blue clay in Madrid. Azarenka pulled out of this Roman event on Wednesday night, citing a right shoulder injury. The Belarusian star pulled out after posting a 55-minute second-round win against Israeli Shahar Peer.

Serena's quarterfinal opponent on Friday will be Italian crowd favorite Flavia Pennetta.

Meanwhile, fourth-seeded Wimbledon champ Petra Kvitova held off Romanian Sorana Cirstea 6-2, 5-7, 6-2 on Day 4. The formidable Czech will face rising German Angelique Kerber on Friday. Kvitova beat Kerber in a quarterfinal in Stuttgart last month.

Eighth-seeded Chinese star Li Na moved on with a 7-6 (7-3), 6-2 victory over South African Chanelle Scheepers in just over an hour-and-a-half.

In other third-round play, a 12th-seeded Kerber handled fellow German Julia Goerges 6-4, 6-1 and Pennetta dismantled Czech Petra Cetkovska 6-0, 6-1. The reigning French Open champ Li will face 14th-seeded Slovak Dominika Cibulkova in the quarters. Cibulkova reached the round of eight via walkover when Azarenka pulled out on Wednesday.

The 2012 Rome titlist will collect $385,000.