"A lot of the players are Darth Vader types to me," said McEnroe in a reference to the half-man, half-machine character from the Star Wars films.

"OK, so they are great athletes but where's the passion, the feeling?", the 45-year-old American was quoted as saying in the BBC's Radio Times magazine.

"It seems to me that even if you, the audience, really dislike the guy on the court, it's better to feel that than to feel nothing," added McEnroe, who will be commentating for BBC television at the Wimbledon championships which start next Monday.

"We need more daring attempts to entertain. The players don't seem happy to be out there."

McEnroe, famous for his controversial, on-court antics during a career in which he won four U.S. Open titles and three Wimbledon crowns, warned that tennis would go downhill unless the attitude of the players changed.

"They (the players) walk around with their heads down. If they're 15th in the world, they're miserable because all they can think about is the 14 people better than them," he said.

"They walk around in a bubble that's all professional sport and no life. We need to work out a way of promoting them, or the game will go down.

"They need to be accessible to their fans. Who wants to watch robots all day?"