New York: Serena Williams' coach Patrick Mouratogloua admitted on Friday that the pressure of pursuing the calendar Grand Slam was "enormous" after the American slumped to a US Open semi-final defeat to Roberta Vinci.

Williams was beaten 2-6, 6-4, 6-4 by the World No 43, who had lost all four previous meetings with the American without claiming a set. "We can say that the pressure of this tournament was enormous and this was a match which she managed less well than the others," said French coach Mouratoglou.

"After her career, what we will remember isn't that she didn't win the Grand Slam. We will say that she is the greatest champion of all time," added the coach, who backed Williams to match and beat Steffi Graf's modern day record of 22 majors.

"There are a lot of players who would love to have 21 Grand Slam titles. She is going to get there. She didn't do it here but she will do it soon."