US Edition 
Wimbledon 2013: Novak Djokovic told to change his shoes as they might give him extra grip on grass
Novak Djokovic’s footwork appeared a touch less secure than in the earlier rounds as he slipped to the ground a number of times in his semi-final against Juan Martin Del Potro on Friday, raising questions that it might have been because he was forced to change the condition of his footwear at the insistence of the All England Club.
This is the second time that a member of the ‘big four’ has been asked to change his shoes during the tournament.
But while Roger Federer's orange-soled shoes fell foul of Wimbledon’s sartorial rule requiring clothing to be “predominantly white”, the club had taken issue with the pimples on the sides of Djokovic’s shoes, which could be seen to give him an advantage.
The rule for grass-court shoes in the Grand Slam Handbook states that “shoes with pimples or studs around the outside of the toes shall not be permitted.
"The foxing around the toes must be smooth”. This was not the case when Djokovic played his first four rounds, as our photograph shows.
Traction on the grass has been an issue for many players throughout the tournament, more than in any other year, and the extra pimples on Djokovic’s shoes would have helped him stay balanced when he moved down into his trademark low squat to retrieve wide balls.
One well-known former player told Telegraph Sport on Friday that he was not surprised that the referee’s office had intervened.
“When you look at how Novak moves into that splits position, and then pushes off with his toes, the pimples on the front of the shoe come into play.
“Other players were interested in what happened with those shoes.
"If Djokovic had been given the all clear, we would have seen more people using them, because this is a sport of tiny margins.
"A fraction more balance and speed on one movement could be enough to make the difference.”
Djokovic is already arguably the best mover in world tennis because of his ability to slide on any surface, where most players are able to slide only on clay.
Andy Murray, who is probably his closest rival in this department, takes small steps to decelerate as he changes direction, which is usually a safer but slightly slower option.