John McEnroe: 'Serena Williams is already an all-time great'

Tennis legend John McEnroe has claimed that Serena Williams is one of the "all-time greatest athletes" in the world.

The 33-year-old American is on course to win the first calendar Grand Slam since 1988 as she prepares for today's semi-final against Roberta Vinci at the US Open.

Williams, who has won the Australian Open, French Open and Wimbledon this season, has 21 Grand Slam titles to her name.

"She's the greatest female player I've ever seen," McEnroe told BBC Sport. "She's proven it to me over and over. I think Serena is one of the all-time greatest athletes, period - man or woman."

The other semi-final at Flushing Meadows in New York is between Flavia Pennetta and Simona Halep.