The American claimed a 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. My shoulder inflammation continues to be a challenge, but I am anxious to return to the court as soon as possible,” said Williams, winner of the last two Cincinnati editions.
She made her decision after a training session at the venue.
Williams was replaced in the draw by Misaki Doi of Japan.
The focus now shifts to German Angelique Kerber, who is challenging Williams for the number one rankling. Williams has topped the table for 183 consecutive weeks and 306 weeks overall.
Source: GNA/News Ghana