Defending champion Rafael Nadal slumped out of the Dubai Open on Thursday, losing 7-6, 6-3 to unseeded Russian Mikhail Youzhny.

The disgruntled Spaniard then slammed the Hawkeye line calling system, accusing it of costing him the first set.

Top seed Roger Federer also struggled but defeated seventh seeded Serb Novak Djokovic 6-3, 6-7, 6-3.

In the semi-finals, Federer will play fifth seeded German Tommy Haas, a 6-0, 6-2 winner of Olivier Rochus of Belgium, while Youzhny will play unseeded Swede Robin Soderling, who beat Fabrice Santoro of France 6-1, 6-7, 6-0.

Second seeded Nadal was furious that, with the score at 6-5 in Youzhny's favour in the tiebreak, a ball from the Russian appeared to be out but was shown to be in when the call was challenged.

"Today I lost the first set in the last point because I saw, and the referee (umpire) knows because he saw the ball outside," Nadal told reporters. "The mark of the ball was still on court and it was outside but in the challenge it was in, so that's unbelievable."

According to the disappointed Spaniard, even umpire Roland Herfel agreed the ball was out, but umpire's are instructed to go with the Hawkeye call.

"I say to him, 'look, the ball is out', and he say 'I know'.

"The truth is put Hawkeye on clay this year and we will see. You will see what's happening with Hawkeye sometimes."

Even Youzhny agreed the ball was out.

"It looked like it was out," said Youzhny. "I saw the mark, but I just took the challenge because it was a very important point. When it showed it was good I was a little bit shocked."

Youzhny's victory was not entirely unexpected, after he won the Rotterdam title last Sunday, while Nadal has not reached a final since Wimbledon, and has not claimed a title since he won Roland Garros in June. Youzhny also won their last meeting, in the quarter-finals of the 2006 US Open.

After claiming the first set in dubious circumstances, Youzhny broke to lead 4-2 in the second by forcing a backhand error. But he then needed seven match points to close out the match as both players lifted what was already a superbly contested battle to a new level.

FEDERER FRUSTRATED

Federer was also frustrated by Hawkeye in the second set tiebreak when Djokovic challenged an in call with Federer leading 6-5. Hawkeye showed the ball was wide, and to add to Federer's frustration Djokovic went on to take the tiebreak 8-6 when the ball hit the net cord. 

Having lost two match points in the tiebreak, Federer composed himself and claimed the only break of the final set to lead 5-3.

"The ball was out, but it's hard to accept, match point," said Federer. "It was a bit of a pity it went three and it had a bitter taste at the end with the netcord and Hawkeye.

"Maybe I should have closed it out earlier and not got to that position."

Haas, who won the Memphis title last week, claimed his 11th straight win, and his 16th victory in 18 matches this season, while Soderling held off a strong challenge from Santoro to reach his third semi-final of the year and his second in two weeks.