The former French Open champion has been struggling with a nasty cough but her experience was crucial as the seventh seed managed to win 7-5 4-6 6-2 in two hours, 37 minutes.
However 2011 Wimbledon champion Petra Kvitova suffered an upset as she crashed out in straight sets to Petra Martic.
The fourth seed had her chances but she could not live with Martic's serve as she slipped to a 6-4 6-4 loss.
Martic's reward is a last 16 tie with Nadia Petrova after the 17th-seeded Russian saw off Simona Halep 7-6 6-0.
Angelique Kerber made short work of wildcard Ayumi Morita as the fifth seed raced to a 6-3 6-4 victory, while No.6 seed Sara Errani overcame Johanna Larsson 6-3 7-6.
Ninth seed Marion Bartoli is also through after her 6-2 7-5 win over Julia Goerges.
Kvitova, 22, admitted she has work to do on her serve after her surprise defeat.
"She has a big serve and it is tough to push her. She kept me on the defensive and running a lot, and that's not my game. She played so well today [Wednesday], very aggressive," she said.
"I need to do some more work on my serve because it didn't help me today [Wednesday]. I didn't get a lot of free points from it. So that will be our main area to work on."
Martic, 21, who reached the fourth round at the French Open, added: "It was my biggest win so far. I beat a top-10 player at Roland Garros this year, but this is a very big win. I'm really happy about this match.
"The last five or six days were really weird. My practices were really bad, I was struggling, I had no confidence."