Serena Williams can't call the semi-final

Serena Williams has predicted an "interesting" semi-final between Sloane Stephens and Victoria Azarenka but can not call a winner.

Williams was knocked out by 19-year-old Stephens earlier today, and although anything is possible, the American knows that Azarenka will provide a tough test.

"Well, I think it's interesting," she said. "I think Sloane, she is in the semis now. That gives anyone a chance.

"I think Victoria is playing well. She's defending champion and a fan favourite. She handles the pressure really well, so we'll see."

Williams has struggled with injury throughout the tournament but refused to blame that for her defeat today.