NEW YORK (Reuters) - Serena Williams, fresh off a third-round upset loss at the Rio Olympics, has pulled out of this week's Western & Southern Open in Cincinnati with a shoulder injury, the world No. 1 said on Monday.

Williams, who had accepted a wild card into the tournament last week after competing at the Olympics, made the decision to skip the key U.S. Open tune-up after a morning practice in Ohio.

"I'm very disappointed I'm not able to compete in the Western & Southern Open in Cincinnati as I was really looking forward to defending my title," she said in a statement on the tournament's website.

"My shoulder inflammation continues to be a challenge, but I am anxious to return to the court as soon as possible."

The decision by Williams comes six days after she tumbled out of the singles competition at the Rio Olympics where she suffered a shock defeat to Ukraine's Elina Svitolina.

The withdrawal by Williams means the tournament second seed Angelique Kerber of Germany will claim the world No. 1 ranking if she wins the event.

Williams, who will be replaced in the draw by Misaki Doi of Japan, has been the world's top-ranked player for 183 consecutive weeks.