Serena Williams will enter the US Open with even more question marks after pulling out of the Western & Southern Open on Monday.
Williams, the two-time defending champion in Cincinnati, cited a right shoulder injury, the same setback that forced her to withdraw from the Rogers Cup in Montreal last month.
"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 in a statement to the tournament. "My shoulder inflammation continues to be a challenge, but I am anxious to return to the court as soon as possible."
Williams did play in the Rio Olympics last week but fell to Ukraine's Elina Svitolina in the third round. During that match -- one in which Williams double-faulted five times in one game late in the second set -- she was visibly bothered by her shoulder. She and sister Venus also lost in the first round of doubles.
With her withdrawal, Williams is now in danger of losing her No. 1 ranking, which she has owned for 183 consecutive weeks, second all time to Steffi Graf's record of 186.
If Rio silver medalist Angelique Kerber wins Cincinnati this week, she would unseat Williams atop the WTA Tour's rankings. Earlier this season, Kerber won the Australian Open, her first major title, and at Wimbledon, she reached the final before running into Williams.
Assuming she is healthy enough to play, Williams will next appear at the US Open, where she has a chance to pass Graf's Open era record of 23 Grand Slam titles.
Williams will be replaced in the Western & Southern draw by lucky loser Misaki Doi of Japan.