Serena Williams, the No. 1-ranked women's player in the world, withdrew from the Western & Southern Open tennis tournament in Cincinnati because of a shoulder injury, the tournament announced on Monday.
Williams dropped out after a morning practice session. She had accepted a wild-card berth into the tournament last week after being upset in the third round of the Olympics.
"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," Williams said, according to Cincinnati.com. "My shoulder inflammation continues to be a challenge, but I am anxious to return to the court as soon as possible."
The shoulder problem also caused Williams to pull out of the Rogers Cup in Montreal last month.
Her withdrawal at Cincinnati gives second-seeded Angelique Kerber an opportunity to take over the No. 1 ranking if she were to win the W&S title.
Williams has been ranked No. 1 for 183 consecutive weeks, which is the second-longest streak in WTA history, trailing Steffi Graf's 186 consecutive weeks.
The 34-year-old Williams has been ranked No. 1 for 306 weeks overall, first reaching the top spot in July 2002.
Japan's Misaki Doi replaced Williams in the draw, receiving Williams' first-round bye.