Serena Williams withdraws from Cinci

Serena Williams has suffered another blow to her US Open preparations, forced to withdraw from Cincinnati with a shoulder injury.

World No.1 Serena Williams has withdraw from the Cincinnati Open with a shoulder injury. (AAP)

Serena Williams has joined Novak Djokovic on the Cincinnati scrapheap, withdrawing from the tournament with a shoulder injury.

Her withdrawal means both the top ranked man and woman in the world have pulled out after poor Olympic campaigns, Djokovic losing in round one to Argentine silver medallist Juan Martin del Potro and Serena falling in round three to Ukraine's Elina Svitolina.

The American claimed the shoulder injury had not healed enough for her to compete at the last major tune-up prior to the start of the US Open in a fortnight.

"I'm very disappointed I'm not able to compete, as I was really looking forward to defending my title," Williams said.

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

Williams was replaced in the draw by Misaki Doi of Japan.

The focus now shifts to Australian Open champion, Germany's Angelique Kerber who is challenging Williams for the world No.1 ranking.