Samantha Stosur has admitted that she will have to come up with a gameplan in order to defeat Flavia Pennetta in the US Open fourth round.

In their six previous meetings, Stosur has won just two sets against the Italian, and the 2011 US Open champion has acknowledged that she may find it difficult to overcome her last-16 opponent.

The 31-year-old is quoted by Rippa.com as saying: "I have never beaten her before. I think we have played six times. I need to knuckle down and be clear with what I want to do and what tactics are going to work for me.

"She can take away some of my weapons, which is why I find it very hard. I will need to do something I have never done before."

While Stosur won the tournament four years ago, Pennetta's best performance came in 2013 when she reached the semi-finals.