Serena Williams in Italian quarters

ROME -- Serena Williams defeated Anabel Medina Garrigues 6-3, 6-1 to extend her winning streak to 16 matches and move into the quarterfinals of the Italian Open on Thursday.

Joining her there will be sister Venus Williams, who upset Samantha Stosur 6-4, 6-3.

Serena called for the trainer while leading 4-1 in the second set and had her left ankle retaped. She easily won the last two games.

"I wanted to loosen the tape, and so I called the trainer to retape," Serena said. "It was too tight."

With the French Open 10 days away, Serena said she's ready.

"I feel good and I have had a lot of practice on clay," she said. "Today I was running so much and I was ready to play and feeling good. I wasn't tired at all."

Serena's quarterfinal opponent will be Italian hope Flavia Pennetta, who routed Petra Cetkovska of the Czech Republic 6-0, 6-1.

For Venus Williams, it was one of her biggest wins since being sidelined six months following the diagnosis of an autoimmune disease, Sjogren's syndrome.

"Every match is a lesson and a challenge for me," she said. "I think that they are all great wins nowadays."

Venus next will face defending champion Maria Sharapova, who beat 2008 French Open champ Ana Ivanovic 7-6 (4), 6-3 after saving three set points in the first.

Wimbledon champion Petra Kvitova struggled before advancing with a 6-2, 5-7, 6-2 win over Sorana Cirstea of Romania. Her quarterfinal opponent will be Angelique Kerber, who defeated Julia Goerges 6-4, 6-1.

French Open champion Li Na eliminated Chanelle Scheepers of South Africa 7-6 (3), 6-2 and will meet Dominika Cibulkova, who advanced when top-ranked Victoria Azarenka withdrew with a right shoulder injury Wednesday.

Information from The Associated Press was used in this report.