LONDON - The Williams sisters are still a force to be reckoned with despite early exits from Wimbledon, nine-times champion Martina Navratilova said after two disappointing women's semi-finals at the All England Club on Thursday.

Venus Williams of the U.S. reacts during her match against Tsvetana Pironkova of Bulgaria at the Wimbledon tennis championships in London June 27, 2011.  [Photo/Agencies]

Defending champion Serena Williams lost to France's Marion Bartoli in the fourth round and older sister Venus followed her out at the same stage after a poor performance against Bulgarian Tsvetana Pironkova.

But Navratilova said the American sisters, who have 20 grand slam singles titles between them, are far from a spent force despite chronic injury problems that sidelined them for most of this year and left the struggling women's game bereft of its two headline names.

"I'm sure that they'll be a force to be reckoned with at the U.S. Open if they stay healthy," Navratilova said at Wimbledon on Thursday. "It's amazing for Serena to be here at all."

The younger sibling only returned to action the week before Wimbledon after 11 months out recovering from a foot injury she suffered after stepping on broken glass followed by life-threatening blood clots in her lungs.

Venus, who has won five Wimbledon singles titles to Serena's four, missed five months with a hip injury.

Thursday's semi-finals were hardly a great endorsement for the women's game with former world number one Maria Sharapova beating wildcard Sabine Lisicki despite serving 13 double faults and Czech Petra Kvitova edging out Victoria Azarenka in a one-dimensional baseline battle.

Serena Williams of the US reacts during her match against Marion Bartoli of France at the Wimbledon tennis championships in London June 27, 2011.  [Photo/Agencies]

Navratilova believes 29-year-old Serena, in particular, can still set the standard.

"I wouldn't write off Serena Williams quite yet," Navratilova said after teaming up with former Swiss great Martina Hingis in the invitational doubles.

"Venus is two years older and it gets harder after 30. I know that well. I think Serena will still be back.

"She'll get in great shape. She still did amazingly well.  I thought both sisters, considering how little they played, they looked sharp. Then they hit a really good player and couldn't cope."

Navratilova refused to criticise the current strength of the women's game, saying a victory for either Williams sister at Wimbledon would not have reflected badly on their rivals despite their injury woes.

"If (Rafa) Nadal had been away for a year, he comes in here, he would be one of the contenders. You'd expect him to get to the quarters or semis. You wouldn't say the men stink," the 54-year-old said.

"I think that is a bit of a double standard. I would look at it as both a tribute to Serena's greatness as well as perhaps lack of women believing in themselves that they can beat them.

"But the intimidation factor that the Williams sisters had has gone down a little bit."