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."
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."
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.