DUBAI, United Arab Emirates (AP) — Serena Williams has withdrawn from the Dubai Championships because of a lower back injury.

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.

Last year, she won the Wimbledon and U.S. Open titles, as well as the Olympic gold medal.

Second-ranked Victoria Azarenka, who beat Williams in the final in Qatar last week, also withdrew from the tournament in Dubai, citing a bone bruise in her right foot.