Defending champ Serena Williams wins her opening match at Cincinnati in straight sets

Serena Williams, of the United States, reacts during a match against Tsvetana Pironkova, of Bulgaria, at the Western & Southern Open tennis tournament, Wednesday, Aug. 19, 2015, in Mason, Ohio. (AP Photo/John Minchillo) (The Associated Press)

Stanislas Wawrinka, of Switzerland, wipes sweat from his forehead during his match against Borna Coric, of Croatia, at the Western & Southern Open tennis tournament, Wednesday, Aug. 19, 2015, in Mason, Ohio. (AP Photo/John Minchillo) (The Associated Press)

Serena Williams pumped her fist after each emphatic point during a two-set victory over Tsvetana Pironkova on Wednesday, a solid start at defending her Cincinnati title.

Williams won the Western & Southern Open for the first time last year, getting one of the few titles that had eluded her. This year, she's using it as a final tuneup for the U.S. Open, where she'll be trying to complete a Grand Slam sweep.

Her sister, Venus, dropped out of the tournament Tuesday night with an illness.

Williams beat Pironkova 7-5, 6-3 in an hour and 29 minutes, fighting off three break points in the first set. Rain fell just as the match ended.

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AP freelance writer Mark Schmetzer contributed to this report.

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