Serena, the reigning Australian Open champion, and Wimbledon champion Venus both endured rugged 2005 seasons because of injury.
The pair's diluted influence was measured in the comparatively meagre number of titles they managed to pocket.
Venus, 25, landed two singles titles with a 37-10 record. Serena was seen at fewer events, finishing with a 21-7 mark and the Rod Laver Arena crown.
Serena, 24, succumbed to a chronic left ankle injury and was finally forced off the circuit after an embarrassing first-round loss in Beijing in September.
Only the second player after Margaret Osborne duPont to save match points twice en route to a grand slam final, Williams wrote off the rest of her season.
She will resume, along with Venus, in a Hong Kong exhibition from January 4-7.
Now ranked 11th, the former world No. 1 underwent knee surgery in October. I didn't hear any news about this. Does anyone know if it's true?
Venus's year was marginally better, highlighted by an emotional rise to a third Wimbledon win, where she saved a match point in the final against Lindsay Davenport.
But she, too, was troubled by a left-knee injury that forced her off the tour.
The Williams sisters are due to face Davenport, Kim Clijsters, Nicole Vaidisova and Sania Mirza in Hong Kong.