World number one Novak Djokovic suffered a surprise quarter-final exit at the Mutua Madrid Open on Friday night.

The Serbian then joined Rafael Nadal in questioning the blue clay court.

Djokovic was undone by fellow countryman Janko Tipsarevic, who racked up 30 winners on his way to an unexpected 7-6 (7/2) 6-3 victory.

Djokovic has struggled against Tipsarevic in the past - he lost to him in London last November - and Friday was no different, with the game's leading player committing 20 unforced errors on his way to a fourth loss of the year.

It was his first on the blue clay, though, and, like Nadal earlier in the week, he questioned the wisdom of the move from the normal red surface.

"I want to forget this week as soon as possible and move on to the real clay courts," he told a news conference after his loss.

"It took me at least a week to try and get used to this surface and somehow find a way to win matches and play a decent level of tennis.

"There is no discussion in my eyes, it's very simple. No blue clay for me."

Tipsarevic's reward, if it can be classed as that, is a last-four meeting with the in-form Roger Federer.

While his peers have floundered on the new surface, the Swiss has made it look easy, and did so again on Friday night as he saw off David Ferrer 6-4 6-4.

The other semi-final pits Tomas Berdych against Juan Martin Del Potro.

Berdych powered to an impressive 6-1 6-2 win over hometown hero Fernando Verdasco - Nadal's conqueror - who was unable to deal with the Czech's sheer power and crashed to defeat in little over an hour.

Del Potro had earlier eased to a 6-3 6-4 victory over Alexandr Dolgopolov.