Former champion Petra Kvitova battled her way through to the Wimbledon quarter-finals with victory over Spain's Carla Suarez Navarro.

Kvitova, the winner in 2011, is the highest seed left in the bottom half of the draw and, with a game made for grass has emerged as a clear favourite to make the final.

But she has not had a good season and has been unconvincing so far this fortnight.

She may well have lost in the third round had bad light not intervened during her match against Ekaterina Makarova.

The eighth-seeded Czech looked better today in a 7-6 (7/5) 6-3 win on Court Three against 19th seed Suarez Navarro, who is unusual among the top women in having a single-handed backhand.

It was still rather a struggle, with Kvitova securing the first break of serve in the 11th game only to be broken back to love, Suarez Navarro producing the shot of the match with a running backhand pass to set up the chance.

The tie-break was scrappy but Kvitova took it when the Spaniard netted a backhand.

Suarez Navarro, who was making a first appearance in the fourth round here, appeared to become distracted by noise from outside the court as she was broken again in the sixth game of the second set.

Kvitova survived two break points in the next game before clinching victory on her first match point when her opponent netted a forehand.

The only American left in the Wimbledon singles, 20-year-old Sloane Stephens, overcame Monica Puig 4-6 7-5 6-1 today and has been tipped for glory by Serena Williams.

"That's very nice of her," Stephens said. "I don't know. I have a couple more matches to go till that happens.

"I am just going to go out and play my hardest and see what I can do."

Stephens will face Marion Bartoli in the quarter-finals tomorrow after the Frenchwoman defeated Italian Karin Knapp 6-2 6-3, with the winner facing 2011 Wimbledon champion Kvitova or Belgian Kirsten Flipkens.

The other half of the draw sees Williams' conqueror Lisicki face Kaia Kanepi, who overcame home favourite Laura Robson 7-6 (8/6) 7-5 on Court One this afternoon.

Expecting a hostile reception from the home fans, the Estonian said: "I think the crowd wasn't that bad actually.

"When she won a point it was a bit louder than normal but they didn't clap when I double-faulted or anything. I think it was really good.

"My serve and baseline game were good today and I guess I will have to hit hard in my next match, that's always important on grass."

Whoever wins tomorrow's duel will face a tough semi-final.

Fourth seed Agnieszka Radwanska reached the final last year and is on course to do so again, although 2011 French Open champion Li Na stands in her way at the quarter-final juncture.

Poland's Radwanska was a 4-6 6-3 6-3 winner against Bulgarian Wimbledon specialist Tsvetana Pironkova, and Chinese hopeful Li took just 55 minutes to crush Italian Roberta Vinci 6-2 6-0.