Top-ranked Serena Williams has withdrawn from the Dubai Tennis Championships because of the lingering effects of an upper respiratory illness sustained during the Australian Open last month.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.

Venus Williams, the defending champion in Dubai, said her sister needed to "regroup and feel better" before playing again. Venus said "it's just very challenging to hit three and four continents in a row. It's a lot when you do all that traveling."

Seventh-ranked Eugenie Bouchard of Canada also withdrew with an arm injury.