Serena Williams put herself back on the winning track in the American's final warm-up event before the U.S. Open with a 7-5 6-3 win over Bulgarian Tsvetana Pironkova in her opening match at WTA Cincinnati.

The defending champion enjoyed a successful return to Cincinnati but Carolina Wozniacki, who Williams beat in the final at Flushing Meadows last year, continued her poor build-up for New York with a straight sets loss to Victoria Azarenka.

Williams prevailed in 90 minutes and showed none of the uncharacteristic struggles that dogged her during last week's semi-final loss to Swiss teenager Belinda Bencic in Toronto.

The world number one, who will aim for a rare sweep of the four grand slam events at the Aug. 31-Sept. 13 U.S. Open, broke Pironkova's serve once in each set and then sealed victory in style with her 15th ace of the second round match.

"There wasn't anything really tactical in particular for me today, I always just try to play my game and be consistent and stay focused," Williams, who improved to 44-2 in 2015, said in a courtside interview.

Fourth seed Petra Kvitova also bowed out, the Czech beaten 7-5 4-6 6-2 by Caroline Garcia, of France, leaving the women's event with just two of the top five seeds still standing.

Garcia will meet 14th seed Elina Svitolina in the next round after the Ukrainian edged out the slumping Eugenie Bouchard 7-6 (6) 7-5 as the Canadian's woeful season continued.

In earlier action, Romanian third seed Simona Halep beat FrenchwomanKristina Mladenovic 7-5 5-7 6-4 while Czech seventh seed Lucie Safarovaadvanced with a 6-4 7-5 win over American wild card Coco Vandeweghe.

Results - round 2

14-Elina Svitolina (Ukraine) beat Eugenie Bouchard (Canada) 7-6(6) 7-5

Victoria Azarenka (Belarus) beat 5-Caroline Wozniacki (Denmark) 6-0 6-4

7-Lucie Safarova (Czech Republic) beat CoCo Vandeweghe (U.S.) 6-4 7-5

1-Serena Williams (U.S.) beat Tsvetana Pironkova (Bulgaria) 7-5 6-3

Anastasia Pavlyuchenkova (Russia) beat Yaroslava Shvedova (Kazakhstan) 2-6 6-3 6-2

8-Karolina Pliskova (Czech Republic) beat Timea Babos (Hungary) 6-4 7-5

Varvara Lepchenko (U.S.) beat Mirjana Lucic-Baroni (Croatia) 7-5 6-4

Belinda Bencic (Switzerland) beat Flavia Pennetta (Italy) 6-1 6-4

3-Simona Halep (Romania) beat Kristina Mladenovic (France) 7-5 5-7 6-4

Jelena Jankovic (Serbia) beat Madison Keys (U.S.) 7-6(4) 6-4

Karin Knapp (Italy) beat Daria Gavrilova (Russia) 6-4 6-1

Caroline Garcia (France) beat 4-Petra Kvitova (Czech Republic) 7-5 4-6 6-2

Sloane Stephens (U.S.) beat Mona Barthel (Germany) 3-6 7-6(1) 6-2

15-Andrea Petkovic (Germany) beat Camila Giorgi (Italy) 5-7 6-3 6-3

Anna Karolina Schmiedlova (Slovakia) beat Irina Begu (Romania) 6-1 6-3

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