MASON, Ohio -- Serena Williams was out, then she was in, now she's out again.
The two-time defending champion and world's No. 1 withdrew from the Western & Southern Open on Monday due to a shoulder injury, putting her long-time top ranking in jeopardy. Williams hit on a practice court at the Lindner Tennis Center and said her shoulder inflammation would prevent her from playing.
"I'm very disappointed I'm not able to compete ... as I was really looking forward to defending my title," Williams said in a statement. "My shoulder inflammation continues to be a challenge, but I am anxious to return to the court as soon as possible."
Besides abdicating her W&S Open title, Williams could lose her 183-week hold on the WTA's No. 1 ranking, second to Steffi Graf's record of 186. Australian Open champ Angelique Kerber will take over No. 1 if she wins at Mason.
It's the same injury that caused Williams to withdraw from the Rogers Cup in Montreal in July.
Williams originally did not enter W&S Open because of her commitment to the Olympics in Rio de Janeiro. But when Ukraine's Elina Svitolin ousted the reigning gold medalist in the women’s singles Round of 16, it left a nice opening in her schedule to return to Mason.
Williams then accepted a wild-card, only to drop out before playing a game.
The top two men's attractions, No. 1 -ranked Novak Djokovic and No. 3 Roger Federer, a W&S seven-time champion, previously dropped out of the Mason tournament due to injuries.
That leaves Olympic and Wimbledon champion Andy Murray and No. 3 Rafael Nadal as the top draws in the men's bracket, and Simona Halep and Petra Kvitova as the top women.