Novak Djokovic complains the blue clay makes it hard to judge the ball's bounce, while Rafael Nadal calls the surface a mistake. Maria Sharapova think it's unique, while a Canadian tennis player has called it slippery "Smurf" clay.
The Madrid Open is garnering extra attention this year as the mixed Masters Series event will be the first top-level tournament to be played on blue clay instead of the traditional red.
Roger Federer and Andy Murray are open to trying the unorthodox surface, while local favourite Nadal has been among the strongest critics.
Milos Raonic called it "Smurf" clay after practicing on it, agreeing with Djokovic that the bounce was lower and the surface was slippery.