NEW YORK, NY, USA - The field in Flushing Meadows has now been whittled down to the last 16 and among those looking to claim a place in the quarterfinals on Sunday are defending champion Samantha Stosur and the in-form Maria Sharapova.
Sunday, Fourth Round
Arthur Ashe Stadium
 Maria Sharapova (RUS #3) vs.  Nadia Petrova (RUS #22)
Head-to-head: Sharapova leads 8-1
Nadia Petrova has to overcome a pretty formidable opponent and, arguably, some even more formidable statistics if she is to reach the quarterfinals in New York; Sharapova has won eight of the nine meetings between the pair - seven of which came on hardcourts - and enters their latest encounter having won 22 of her last 24 on tour.
If this was not daunting enough, the World No.3 has dropped a measly seven games thus far, looking particularly menacing during a quick-fire, 61 61, triumph over Mallory Burdette in the last round. However, Petrova's progress has been none too shabby either; she is yet to drop a set and if her serve continues to flow she is more than capable of troubling her fellow Russian.
Laura Robson (GBR #89) vs.  Samantha Stosur (AUS #7)
Head-to-head: first meeting
After ending Kim Clijsters' 22-match New York win streak in the second round and defeating a red hot Li Na in the third, things are not about to get any easier for Laura Robson. Next up for the 18-year-old is the small matter of a date with defending champion Samantha Stosur. Although, after the poise - and shotmaking - displayed thus far, a third upset is by no means beyond her.
How Robson gets to grips with Stosur's heavy topspin, particularly on the forehand, is likely to play a pivotal role in the outcome. Going by her performances on court, and laid back mood off it, the Australian seems to be reveling in the role of reigning champion and it will take something truly special to dethrone her.
 Marion Bartoli (FRA #11) vs.  Petra Kvitova (CZE #5)
Head-to-head: Bartoli leads 2-1
Marion Bartoli picked up just two games when she met Petra Kvitova in Montréal a few weeks back, describing her opponent's performance after the match as 'extraordinary.' Kvitova went on to pick up the title in Canada and repeated the trick in New Haven to cap off a memorable summer on the cement, placing her among the favorites for the title this fortnight.
But while recent form - Kvitova's three first week wins have taken her record since the Olympics to 15-1 - suggests only one winner, Bartoli is often at her most dangerous when the odds are stacked against her. Bartoli impressed during a tricky third round with fellow Frenchwoman Kristina Mladenovic, and, despite what happened in Montréal, she knows victory is not beyond her, having won her first two career meetings with Kvitova.
Around the grounds?
The day's other fourth round match pits No.1 seed Victoria Azarenka against one of the tournament's surprise packages, Anna Tatishvili. Meanwhile, in the doubles several of the favorites are in action, including top seeds Liezel Huber and Lisa Raymond and their Italian rivals Sara Errani and Roberta Vinci.