Serena Williams pulls out of Dubai Tennis Championships

Serena Williams has withdrawn from this week's Dubai Tennis Championships because of a lower back problem.

The 31-year-old, who on Monday became the oldest woman to top the women's world rankings, had been one of the favourites for the title.

It means the tournament will be without the world number one and number two after Victoria Azarenka pulled out through injury on Tuesday.

Azarenka, unbeaten in 2013, has been hampered by a foot problem.

The 23-year-old Belarusian sustained the injury on her way to winning last week's Qatar Open, when she beat Williams in the final.

Williams was set to face Marion Bartoli in the second round, but the Frenchwoman will now meet Caroline Wozniacki in the next round.