World number one Novak Djokovic was buried by 21 aces as big-serving Croatian Ivo Karlovic clinched a 6-7 (2/7), 7-6 (8/6), 6-4 victory to reach the Qatar Open semi-finals on Thursday.

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Top seed Djokovic appeared affected by a steady breeze in the Qatari capital, which did not prevent Karlovic, the six-foot 11-inch, 35-year-old from taking his aces total to 67 in three matches.

Djokovic was nevertheless three times within two points of winning the match in the second set tie-breaker, before dropping serve for 3-4 in the final set as a visibly encouraged Karlovic came up with some fine forehand winners.

The top seeded Serb became sufficiently exasperated to fling down his racquet in the second set and then to sling it along the floor in the third.

He never looked likely to make a break back, eventually settling instead for congratulating Karlovic who claimed a second win over the world number onwe to add to his 2008 Madrid Masters victory.

"I was feeling good even though it was windy," said Karlovic.

"I know he wasn't able to show his full range (of shots) because of the wind. I was relaxed and didn't feel I had to do anything extraordinary. I just had to focus on my game. So I guess I was luckier than him. But I am very happy that I won."

The result was even more surprising than the first round defeat of the titleholder Rafael Nadal, whose fitness problems had been well documented, and who was a long way from his best against German qualifier Michael Berrer.

True, Djokovic had been suffering from a fever prior to the tournament and had considered pulling out, but by the time he had dismissed Dusan Lajovic and Sergiy Stakhovsky for the loss of only six games in four sets he seemed to have recovered well.