Top-ranked Serena Williams has withdrawn from the Dubai Tennis Championships because of the lingering effects of an upper respiratory illness sustained during the Australian Open last month.
Williams suffered from a bad cough in her last few matches en route to winning her 19th career Grand Slam title at the Australian Open.
Last weekend, Williams played one singles match for the United States in the Fed Cup against Argentina in Buenos Aires but was replaced by CoCo Vandeweghe on the second day.
Defending champion Venus Williams beat an opponent half her age on Monday, winning 6-1, 6-2 against 17-year-old Belinda Bencic of Switzerland in the first round of the Dubai Tennis Championships.