London: Nine-time Wimbledon champion Martina Navratilova believes it would be foolish to write off the Williams sisters as a spent force and backed the American sisters to star at the US Open.

Serena Williams missed 11 months of action after injuring her foot in a freak accident at a German restaurant after winning her fourth Wimbledon title last year and then battled life-threatening blood clots on her lungs.

Her lack of matchplay was obvious when she crashed out of the fourth round at Wimbledon on Monday with big sister Venus also exiting the tournament on the same day.

Serena will celebrate her 30th birthday in September, but Navratilova insists the great American is not ready to shuffle off into retirement.

"She'll get in great shape. She still did amazingly well.

I thought both sisters, considering how little they played, they looked sharp," said Navratilova.

"I'm sure they'll be a force to be reckoned with at the US Open if they stay healthy. I wouldn't write off Serena Williams quite yet. I think Venus is two years older and it gets harder after 30. I think Serena will still be back."

Meanwhile, Navratilova backed fellow Czech Petra Kvitova not to be overwhelmed by favourite Maria Sharapova in Saturday's Wimbledon women's singles final.

Kvitova is bidding to become the first Czech champion since Jana Novotna in 1998 and first left-handed winner since Navratilova in 1990.

"She's riding a big wave. She was in the semis last year.

She's in the top 10, still improving. I've been saying all along the next wave of players will become better all around players," said Navratilova.

"They'll come to the net more. She likes to move forward.

She's not just going east/west, she's looking to go north. It's nice to see that."