Rafael Nadal on Friday blasted the decision to play next month's Madrid Masters, a key tournament in the build-up to the French Open, on blue clay rather than the traditional red.
"My opinion, it's a mistake," said the Spanish world number two.
"You are in the middle of the clay court season, and the clay here in Europe is red.
"Madrid is the only tournament you are playing at high altitude, and then now you are putting a different colour of clay. Madrid is big enough not to need this promotion. The history of the clay court season is on red, not blue."
World number one Novak Djokovic also added his concern, saying it was the wrong time to experiment with playing conditions.
"Sometimes change is good. I like innovative and creative people," said Djokovic after reaching the Monte Carlo Masters semi-finals.
"But, on the other hand, it's going to be the only blue clay court tournament in the world. To be honest with you, as far as I know, most of the top players I talked to, nobody has agreed."
Djokovic named Roger Federer and Nadal as sharing his doubts about the wisdom of the publicity-generating experiment from master showman Ion Tiriac.
"I've never played on blue clay, Rafa hasn't, Roger hasn't. We're going on there and we're going to play for the first time ever.
"We don't even know if it's a natural blue clay because natural clay is a red clay."
Djokovic added: "I'm not really too happy about it. It's going to be interesting to step on the blue clay. All credit to the tournament. I'm not blaming them. They fight for their own."
But the Serb said that changes need to be made to the rule which allowed former president of the ATP, Adam Helfant, to back the change in the surface.
"There is a certain rule that the president is able to make decisions by himself without having players agree. That rule has to be changed because it's not fair. That's what happened last year. That is why Madrid has a blue clay."