The women's singles event of the U.S. Open is full of surprises as five doubles players have advanced to the quarterfinals at Flushing Meadows New York.

Five of the remaining eight quarterfinalists of the singles competition are also women's doubles players and all five of them are not even teamed up together.

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World No. 1 Serena Williams comes out on top of the list that is partnered with sister Venus and are now the favorites of winning the event after top seeds Sara Errani and Roberta Vinci, and second seeds Peng Shuai and Hsieh Su-wei failed to reach the quarters.

Williams is set to face Flavia Pennetta in the next round who is another doubles player that is playing alongside Martina Hingis.

Pennetta and Hingis are also in the last eight teams in the doubles competition and will be playing fifth-seeds Kveta Peschke and Katarina Srebotnik.

The third doubles player who also reached the quarters in both singles and doubles is Russian Ekaterina Makarova and is teamed up with fellow compatriot Elena Vesnina.

Makarova is on to a tough challenge in the singles against last year's runner-up Victoria Azarenka while at doubles against the Williams sisters.

Sara Errani is out to duplicate her majors' performance by entering the semis, that's if she surpass the talented 10th-seeded Caroline Wozniacki.

And finally, Peng Shuai who is having a remarkable tournament, defeating fourth-seeded Agnieszka Radwanska in the second round, 28th-seed Vinci in the third and Lucie Safarova last Monday to end up a clash with another top seed crusher 17-year-old Belinda Bencic of Switzerland.

The top three players of Williams, Pennetta and Makarova are demanded of performing more since they have little time to rest between competitions.

Pennetta who won her singles match against Casey Dellacqua Monday morning and competed in the doubles in the afternoon along with Hingis is a prime example of fatigue being a factor for she is set to compete again Tuesday in the doubles.

Tagsserena williams, Peng Shuai, Flavia Pennetta, Ekaterina Makarova, Sara Errani