MASON, Ohio (AP) Serena Williams withdrew from the Western & Southern Open with a sore shoulder on Monday, putting her No. 1 ranking in jeopardy.
Williams originally planned to skip the Cincinnati tournament, which comes right after the Olympics.
When she was upset in Rio, she accepted a wild card at Cincinnati and practiced on Monday morning.
Later in the day, the two-time defending champion informed the tournament that she was withdrawing because of an inflamed shoulder.
''My shoulder inflammation continues to be a challenge, but I am anxious to return to the court as soon as possible,'' she said in a statement.
Williams is ranked No. 1 for the 306th week overall in her career and her 183rd in a row. It's the second-longest streak in WTA history, trailing Steffi Graf's 186 consecutive weeks. She first topped the rankings in July 2002 and, at age 34, is the oldest woman to hold the top spot since the computer rankings were introduced in 1975.
Second-ranked Angelique Kerber will take over the top spot if she wins the tournament.
Williams' withdrawal also leaves the tournament without its top seeds in both brackets.
Novak Djokovic also was upset in Rio and then withdrew from Cincinnati because of a sore left wrist. Defending champion Roger Federer is sidelined for the rest of the season while he rehabilitates his surgically repaired knee.
Without Djokovic and Federer, the men's draw is wide open. Fourth-ranked Stan Wawrinka sees it as an opportunity, if his back can hold out. He withdrew from the Olympics because of a sore back.
''I had to take some days for rehab,'' the 31-year-old from Switzerland said. ''I took eight days off from tennis and started playing last Monday.''
Wawrinka, whose best Cincinnati finish was the 2012 semifinals, showed up for practice on Thursday and prepared to resume a season that had included three championships. He reached the French Open semifinals and the Rogers Cup at Toronto.
Wawrinka, who is 31-11 this season, found time between practice sessions to watch the Olympic tennis tournament in Brazil.
''It was not easy to not be there, but it would've been way too much to fly to Rio,'' he said.
Wawrinka is seeded second at Cincinnati behind Olympic singles champion Andy Murray and ahead of Spain's Rafael Nadal, who finished fourth at the Olympics in his first tournament since injuring his left wrist at the French Open.
''It was great to see him back,'' Wawrinka said of Nadal, a possible Cincinnati semifinal opponent. ''I think that was his best hardcourt tournament this year.''
Wawrinka has a first-round bye and is scheduled to meet American wild-card entrant Jared Donaldson, who defeated Spain's Nicolas Almagro 6-3, 7-6 (2) in a first-round match on Monday.
In another first-round match, unseeded Julien Benneteau upset 10th-seeded Spaniard David Ferrer 6-4, 2-6, 6-4. Benneteau lost 6-3, 6-2 in the 2014 Cincinnati semifinals.
In the first match of the women's draw, Jelena Ostapenko of Latvia needed 2 hours, 40 minutes to outlast Slovakia's Anna Karolina Schmiedlova 1-6, 7-6 (6), 7-6 (5).