Serena pulls out in Dubai

After returning to the top of the women's rankings this week, Serena Williams decided to pull out of the Dubai Tennis Championships because of a lower back injury.

The reigning Wimbledon, U.S. Open and Olympic champion actually took over the No. 1 ranking last week in Qatar, marking the sixth time in her career she has been the top-ranked woman in the world. In doing so, the 31-year-old Williams became the oldest-ever No. 1.

Williams was scheduled to meet former Wimbledon runner-up Marion Bartoli on Wednesday.

On Tuesday, former No. 1 and 2010 runner-up Victoria Azarenka pulled out of Dubai, citing a foot injury. The Australian Open titlist beat Williams in last week's marquee 1-versus-2 finale in Doha.