Williams took over the No. 1 ranking last week in Qatar, the sixth time in her career she has been the top-ranked player in the world. She lost in the quarterfinals of the Australian Open last month after injuring her back.
Last year, Williams won the Wimbledon and U.S. Open titles, along with the Olympic gold medal.
Second-ranked Victoria Azarenka also withdrew from the tournament in Dubai, citing a bone bruise in her right foot. Azarenka beat Williams in the finals in Qatar.