The Williams Sisters are projected to drop in the rankings after the Australian Open, with Serena tumbling to a nearly three-year low.
Injuries have hobbled the once dominant pair.
Younger sister Serena, 29, owns 13 Grand Slam trophies, including from last year's Australian Open and Wimbledon. But, shortly after her win at the All England Club, Serena cut her foot on broken glass at a restaurant and sustained an injury that has kept her sidelined ever since.
Serena is currently ranked No. 4 but when the new rankings come out, she is projected to drop to at least No. 13, the WTA said. It would be her lowest ranking since March 2007 when she slipped to No. 18.
Even before the bad news was made public, Serena tweeted from home Sunday that she wasn't feeling well from a cold.
''My throat feels raw. every time I take a sip of sumthin it feels like i just swallowed 20,000 mini knives,'' said Serena, who keeps a running public conversation with herself on Twitter.
Venus, 30, withdrew four minutes into her third-round match in Melbourne due to an ailing hip muscle. It was the first time in 251 Grand Slam matches she had retired from a match at a major.
The No. 5-ranked Venus will exit the top five for the first time in a year, the WTA said. In January 2010, Venus was ranked No. 6.
The older Williams sister has won seven Grand Slams, most recently at Wimbledon in 2008.
If No. 1 Caroline Wozniacki reaches the semifinal she will retain her top ranking.
