World number one Novak Djokovic has threatened to pull out of next year's Madrid Open following his quarter-final defeat to Janko Tipsarevic.
The Serbian has joined Rafael Nadal in condemning the controversial blue clay surface after a number of players have struggled to hold their footing on court.
"In 2013, if they play on blue [clay], they can have their own tournament but I am not coming for sure," Djokovic told reporters.
"It is simple - no blue clay for me. The test has failed.
"I want to forget this week as soon as possible and move on to the real clay courts. Here you can't predict the ball bounce or movement."
The defeat against his fellow Serb was Djokovic's fourth of the season.
