A big puff of pizzazz seeped out of the Cincinnati Open on Thursday as its top seed Caroline Wozniacki slipped out and star attraction Serena Williams quit the tournament, along with third seed Victoria Azarenka.

Fans and organisers were left deflated with 13-times Grand Slam winner Williams and third seed Azarenka both having been scheduled to play on centre court as part of a busy schedule alongside the ATP men's event.

Williams pulled out citing a right toe injury -- but also noting her need to get some rest before the US Open.

Azarenka then withdrew due to a right hand injury which she didn't want to risk just two weeks from the final Grand Slam event of the year in New York.

Those fans of the women's game who did manage to see one of the favourites witnessed an upset with Wozniacki looking well below her best against 19-year-old American Christine McHale.

With golf sensation and friend Rory McIlroy in attendance, the Danish top seed never looked close to her best as she fell 6-4, 7-5 to the 76th ranked McHale.

The defeat was the first time Wozniacki lost to an opponent outside the top 75 since 2009 and extends a worrying spell of form for the Dane.

Wozniacki, who suffered early round exits in her last two tournaments, handed victory to McHale with an awfully-timed stroke which looped way over the baseline.

The 21-year-old heads to the US Open later this month in search of her first Grand Slam title.

Serbian Ana Ivanovic's disappointing season continued with a 6-3, 7-6 loss to Russia's Nadia Petrova, but another Serbian former number one, Jelena Jankovic enjoyed a 4-6, 6-3, 6-1 win over China's Zheng Jie.

Germany's Andrea Petkovic advanced with a 6-2, 6-4 win against Sweden's Sofia Arvidsson.