Maria Sharapova’s chances of stopping Serena Williams win a fourth grand slam this year are in doubt after the Russian admitted she was still short of fitness a day before the start of the US Open.

Sharapova is struggling with a muscle strain in her right leg and has not played a competitive match since losing to Williams in the Wimbledon semi-finals just over seven weeks ago.

The five-time major champion was forced to withdraw from the Rogers Cup in Montreal earlier this month and has only been able to do limited training in the build-up to the year’s last major tournament.

“You have to be realistic and limit your expectations but it’s not easy,” Sharapova said.

“As a competitor, when you get on the court you want to do the best you can.

“I will try limit my expectations but it’s not easy.

“I’m feeling better, it’s just a time thing, dealing with these niggling issues. It’s just a process.”

Sharapova could meet Serena Williams in the semi-finals at Flushing Meadows.

Williams is hoping to become the first female player to win a ‘calendar grand slam’ since Steffi Graff in 1988 and the fourth to achieve the feat in history.

“In many ways it’s just a huge bonus for her to be in this position,” Sharapova said.

“She’s got so many great memories and victories in New York and being back for such a special occasion is a big story.

“Has she got a great chance? I think you could safely say that, yes.”