Caroline Wozniacki of Denmark reacts during her match against Dominika Cibulkova of Slovakia at the Wimbledon tennis championships in London, June 27, 2011. [Photo/Agencies]

LONDON - It took less than two brutal hours at Wimbledon on Monday to disprove the theory that Serena and Venus Williams could march back to continue their domination of the grasscourt slam having hardly swung their rackets in anger for months.

First defending champion Serena was bundled out in the fourth round 6-3 7-6 by livewire Frenchwoman Marion Bartoli then five-times champion Venus was destroyed 6-2 6-3 by Bulgarian Tsvetana Pironkova.

World number one Caroline Wozniacki's shock 1-6 7-6 7-5 loss to Slovakia's Dominika Cibulkova came in the immediate aftermath of the double Williams bombshell and left 2004 champion Maria Sharapova as the clear favourite for the title after the Russian fifth seed overpowered China's Peng Shuai 6-4 6-2.

Serena, who returned from 11 months out with a lacerated foot and then life-threatening blood clots just in time for Wimbledon, spent the first three rounds shaking off the rust and appeared to be rediscovering the firepower that has brought the 29-year-old 13 grand slam titles.

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

Once again she scrapped like an alley-cat but the unorthodox Bartoli, beaten by Venus in the 2007 final, proved a match too far for the champion whose hopes of three consecutive titles evaporated in oppressive heat on Court One.

After defeat, though, she fired out an ominous warning to her rivals and any one preparing to write her off.

"Even today I lost, but I was able to kind of hang in there and play tough," Williams, who was watched by friend and soul diva Beyonce, told reporters. "And I can only get better. And that can potentially be really scary, because I can only go up from here and I can just do so much more."

Bartoli appeared to be playing with fire at times, jumping around inside the baseline as her opponent wound up her mighty serve and fist-pumping towards Williams after big points.